1. The age at puberty in the females of most
domesticated breeds of sheep is 6 to 9 months and
ewes are fertile at this age. There is evidence
to suggest that certain of the more primitive
breeds do not reach puberty until the second
autumn after birth.2. In the ewe, sterility does not result from in-
:creasing age so long as general bodily health is
retained. It is suggested that the gradual
decrease in fecundity during later life is due to
declining bodily health.3. A brief account is given of the growth and
differentiation of the reproductive organs from
birth to sexual maturity.4. There is a well defined prepubertal phase of
rapid growth in the uterus and ovaries. The
uterine glands are formed during this phase.5. The fundamental nature of the mammalian breeding
season is emphasised and its evolutionary
significance is discussed.6. A survey of published data regarding the breeding
season in domesticated sheep has been made.7. The duration of the breeding season in Scottish
sheep has been determined by direct observation
of a small flock and by the observation of
ovarian activity in slaughtered ewes.It was found that, in mature Scottish sheep
generally, the breeding season lasts from about
the middle of October until about the end of
February and includes about 7 or 8 dioestrous
cycles. Exceptionally, it may last for as much
as 7 months and include 12 or more cycles.8. In ewe lambs 6 to 9 months old the breeding season
commences about a month later and ends about a
month earlier than in mature animals.9. The existence of breed differences affecting the
duration of the breeding season is noted and
evidence is produced to show that there are no
considerable differences between the various
Scottish breeds in this respect.10. Minor differences in nutritive conditions do not
affect the duration of the breeding season.11. The average date of onset of the breeding season
is subject to breed differences: most of the
Scottish breeds do not differ greatly from one
another in this respect but the Shetland breed
comes into season fully a month later than the
others.12. There is evidence to suggest that, in breed
crosses, "early breeding" behaves as a dominant
or incompletely dominant genetic character.13. Nutritive "flushing" hastens the onset of the
breeding season. It is suggested that an
improvement in nutritional conditions converts
phases of follicular maturation which would not
otherwise be accompanied by the manifestation of
heat into normal heat periods, and does not
hasten follicular maturation (see also 32).14. The effects of climate and environment upon the
limits of the breeding season are discussed and
the absence of comparative data noted.15. The influence of seasonal changes in the environment upon the onset of the breeding season is
discussed in relation to the cause of the
restriction of the breeding season, and the
tentative hypothesis is advanced that reproductive
activity in the ewe is limited primarily by the
diurnal duration and intensity of daylight, but
that warm climates are conducive to high
reproductive activity in the sheep as in other
mammals.16. The duration and intensity of light are thought
to influence reproductive activity through the
anterior pituitary gland. Inhibition of
pituitary activity by substances elaborated by
photochemical reactions in the eye during anoestrum is suggested as a possible mechanism.17. The results of an experiment designed to test
the hypothesis that light is the principal factor
in controlling reproductive activity in the ewe
are presented and discussed. The results of
this experiment were suggestive but inconclusive.18. The conclusion of previous authors that wild
sheep are monoestrous is criticised.19. The durations of 63 normal dioestrous cycles and
of 77 normal oestrous periods have been determined. The mean duration of the dioestrous cycle
was 16.4 days (mean deviation ± 0.8 days) the
modal duration was 16.5 days and the range of
variation was from 15 to 18.5 days. The mean
duration of oestrum was 36 hours (mean deviation
± 22 hours), the modal duration was 28 hours and
the range of variation was from 3 to 84 hours.I20. The mean duration of the dioestrous cycle deduced
from all available data and representing 697
cycles was 16.8 days, and the chief modal duration
was 17 days. The range of variation was from 6
to 68 days but the distribution was multimodal.21. It is suggested that multiplication of the
cycle duration takes place as a result of the
occurrence of normal ovulation without the
exhibition of the mating instinct (see also 32)22. The influences of internal and external factors
upon the dioestrous cycle are discussed. It is
thought that breed,age, climate, environment and
nutrition may all affect the duration and
periodicity of oestrum slightly but conclusive
evidence is lacking. Sterile service does not
affect the cycle in any way.23. Removal of the corpus luteum appears to cut
short the dioestrous cycle but the evidence for
this is slight.24. Long heat periods are more intense than those
which are of short duration.25. There is no correlation between the duration of
heat and the duration of the preceding or
subsequent dioestrous cycle, but there is a
negative correlation between the duration of heat
and the duration of both preceding and subsequent
interoestrous periods.26. The end of heat is more labile than its
commencement.27. It is postulated that the intensity and duration
of heat are determined by the amount of heatcausing hormone released during the maturation of
the follicle and that heat ceases when the amount
of this hormone in the blood falls below a certain
threshold value. The time of ovulation is not
thought to affect the duration of heat unless it
is definitely delayed.28. The changes in the behaviour and in the external
genital organs of the ewe during the dioestrous
cycle are described.29. It is concluded that it is possible to determine
the limits of the heat period only by observing
the mutual behaviour of the ram and ewe.30. Heat comes on very rapidly and disappears slowly.31. The gross changes in the ovaries during anoestrum
and the dioestrous cycle are described. During
anoestrum follicular maturation and rupture are
in abeyance. Each dioestrous cycle is associat-
:ed with the growth and rupture of a crop of
Graafian follicles, with the development from the
ruptured follicles of corpora lutea, and the
subsequent atrophy of these.
32. Conclusive evidence is presented to show that
normal maturation and rupture of Graafian
follicles and the development of corpora lutea
occur prior to the commencement of the breeding
season and occasionally during the breeding
season without the exhibition of the mating
instinct.
33. Interoestrum is associated with the development
of corpora lutea. Full development of the
corpora is attained by about the eighth day after
the commencement of heat. Atrophy sets in on
about the sixteenth day of the cycle and is soon
followed by the onset of a new heat period.
34. Oestrum is associated with the very rapid
increase in size of one or more Graafian
follicles. These rupture about mid -oestrum.
35. Growth curves illustrating the development of
the Graafian follicle and the development and
atrophy of the corpus luteum during the dioestrous
cycle are given.
36. The diameter of the follicle at the time of
rupture varies considerably but it is usually
about 10 millimetres.
37. The changes in the follicle immediately before
rupture are described.
38. The literature on the subject of the cause of
ovulation in mammals is discussed, and it is
concluded that although the stimulus to
follicular maturation and dehiscence probably
originates from the pituitary gland, the immed-
:iate cause of rupture is the accumulation of
fluid, probably chiefly blood plasma, within the
follicle.
39. Published data regarding the time of ovulation in
the ewe are reviewed and it is concluded that
ovulation in those breeds for which data are
available takes place about 30 hours after the
commencement of heat.
40. Reasons are given for the belief that ovulation
occurs about 18 to 24 hours after the commencement of heat in Scottish sheep.
41. Ovulation is entirely spontaneous and there is no
evidence that it can be hastened by coitus.
42. The view is adopted that heat is caused by a
hormonic stimulus emanating from the ovaries
during the time when follicles are maturing,
but that the hormone produced by the ovary does
not itself stimulate the nervous system but is
effective through a further hormonic link.
43. It is thought that the immediate regulator of
oestrous periodicity is the corpus luteum and it
is postulated that the life of the corpus luteum
is determined by the duration of secretion,
probably by the pituitary, of some substance
necessary for the life of the luteal cells, or
perhaps by the rhythmic secretion of some
substance inhibiting luteal growth.
44. The anatomy and histology of the uterus, cervix
and vagina are described briefly, and a detailed
account is given of the modifications in the
structure of these organs associated with the
dioestrous cycle and with anoestrum.
45. The uterus is slightly congested during oestrum
and early metoestrum; slight extravasation of
blood sometimes occurs in late oestrum or early
metoestrum and there may be haemorrhage into the
uterine lumen, but the amount of blood discharged
is rarely sufficient to be detectable macroscopically. During anoestrum the uterus becomes
anaemic.
46. The uterine stroma becomes oedematous during
oestrum and early metoestrum.
47. During anoestrum the epithelium of the uterus and
the uterine glands is reduced to cubical or low
columnar form and the lumina of the glands are
occluded. The amount of cytoplasm in the
epithelial cells is greatly reduced and it is non - granular and non -vacuolated. There is no
secretory activity.
48. Oestrum is marked by a phase of secretory and
degenerative changes in the epithelia of the
uterine surface and glands. The limits of this
phase are subject to considerable variation.
49. During interoestrum these epithelia hypertrophy
and secretory activity is in abeyance.
50. There is a phase of mitotic activity in the
uterine epithelia lasting from about the time of
ovulation until about the sixth day of the cycle.
During dioestrum and early oestrum mitotic
activity is in abeyance.
51. The stroma nuclei hypertrophy slightly during
dioestrum. Mitotic division of the stroma
nuclei is most common during the same phase of
the cycle in which mitosis occurs in the epithelia,
but is always rare.
52. Lymphocytes are present in the uterine mucosa
during all phases of the dioestrous cycle and
their number shows no cyclical variation. They
are distributed throughout the mucosa and pass
through the epithelia into the lumina of the
uterus and uterine glands. Leucocytes are found
in the uterine mucosa only during pregnancy and
the puerperium.
53. The uterine mucosa is usually deeply pigmented,
especially on the cotyledons. The pigmentation
is due to the presence of a subepithelial layer
of melanoblasts. Pigmentation bears no relation
to the stages of reproductive activity except
that it largely disappears in pregnancy. It is
found in prepubertal and in foetal animals. The
melanin is identical with that in the skin. The
Fallopian tubes are similarly pigmented.
54. There appear to be breed differences in the
occurrence of uterine pigment but pigment is
found in the uteri of all common Scottish breeds
of sheep.
55. It is suggested that the pigment possesses no
functional significance.
56. Other pigmented cells which are not dendritic and
in which the pigment is non -granular are found
during involution of the uterus. The pigment in
these cells is probably of haematogenou s origin.
57. The vagina, vestibule and vulva become congested
and oedematous during early oestrum. Haemorrhage
does not take place.
58. Relaxation of the vaginal muscles occurs in
oestrum.
59. There are no secretory cells in the vaginal
epithelium.
60. Proliferation of the stratum germinativum of the
vaginal epithelium is continuous during the
dioestrous cycle but there is a slight increase
in the frequency of mitosis during early oestrum.
Mitoses are rare during anoestrum.
61. Intensive desquamation of the superficial layers
of the vaginal epithelium takes place late in
oestrum or early in metoestrum. At this time
4 or 5 layers of cells may be cast off. Slight
desquamation takes place continuously throughout
the dioestrous cycle and in anoestrum.
62. The thickness of the vaginal epithelium is
greatest during oestrum, is reduced by the
metoestrous desquamation and increases slowly
through dioestrum. In anoestrum the epithelium
is reduced below the metoestrous level.
63. Partial keratinisation of the superficial layers
of the vaginal epithelium may occureat any time
during the dioestrous cycle and in anoestrum
but is essentially characteristic of late oestrum
and metoestrum. Keratinisation is always
regional.
64. Desquamation of the vaginal epithelium is not
caused by keratinisation.
65. Lymphocytes are constantly present in the
vaginal mucosa. They show no variation in
number during the dioestrous cycle but are reduced in number during anoestrum.
66. Invasion of the vaginal mucosa and lumen by
polymorphonuclear leucocytes may occur at any
phase of the dioestrous cycle and in anoestrum
but the tendency towards leucocytosis is greatest
during dioestrum and least during anoestrum.
Iiechanical stimulation of the vaginal mucosa
greatly increases the tendency towards leucocytec.
infiltration.
67. Changes in the character of the vaginal mucosa
similar to those which occur during the dioestrous
cycle take place during the last month of anoestrum in association with the cycles of
follicular maturation and rupture which occur at
that time, unaccompanied by the exhibition of
"heat".
68. The glandulae vestibulares majores are vestigial
and are thought to be devoid of functional
significance.
69. The muscles of the cervical canal relax during
oestrum.
70. The epithelium of the cervical canal is composed
entirely of mucus-secretory cells.
71. Secretion of mucus takes place continuously
throughout the dioestrous cycle but is most
intensive during oestrum. There is no secretion
of mucus during, anoestrum but slight amounts may
be found in the epithelial cells.
72. The mucus secreted during interoestrum is
tenacious and remains in the cervical canal,
forming a rudimentary cervical plug.
73. During oestrum the cervical plug liquefies and
the mucus flows into the vagina. The mucus
secreted at this time is also fluid.
74. The liquefaction of the cervical mucus commences
shortly before the onset of heat but the highest
concentration of mucus in the vagina does not
occur until heat is well advanced.
!75. The value of the vaginal smear as a diagnostic
measure in determining the stages of reproductive
activity is discussed.
76. There are no gross changes in the mammary glands
during the dioestrous cycle.
77. A brief description of the changes in the
reproductive organs during pregnancy is given
and the changes in the uterine mucosa associated
with the early development of the foetus and
attachment of the foetal membranes are described
in detail.
78. The development of the foetus is very slow until
about the 15th day and thereafter very rapid until
about the 27th day.
79. Attachment of the foetal membranes to the uterine
mucosa commences to take place on about the 26th
day after coitus: it does not commence until
the allantois has become attached to the chorion
and the uterus is distended by the expanding
allantois. The embryo is in an advanced state
of development before attachment begins.
80. Slight atrophy of the corpora lutea occurs on
about the 15th day of pregnancy. They recover
and become more strongly vascularised by the
18th day.
81. At about the 15th day secretory and degenerative
changes commence to occur in the epithelium of
the uterus and the epithelium of the uterine
glands. Secretory activity continues throughout
pregnancy in those parts of the surface epithelium
which are in contact with the foetal membranes
but where the foetal membranes are not in contact
with the mucosa the epithelium hypertrophies
without secreting.
82. It is postulated that the presence of the
foetus does not affect the normal dioestrous
cycle until atrophy of the corpus luteum sets
in. The secretory and degenerative changes in
the uterine mucosa which then occur are thought
to provide the foetus with the materials for
rapid growth and the expansion of the foetal
membranes is thought, by stimulating the uterine
mucosa, to arrest the atrophy of the corpus
luteum.
83. The epithelium covering the cotyledons is denuded,
by the phagocytic and digestive action of the
foetal trophoblast. There is also slight
erosion of the stroma and the surface capillaries
break down.
84. The attachment of the foetal membranes is a
reciprocal process in which both a burrowing
action of the foetal trophoblast and hypertrophy
of the stroma are involved.
85. The formation of the decidua is largely effected
by hypertrophy of the stromal cells and by
expansion of the intercellular spaces but there
is also proliferation of the stroma.
86. Modification of the cotyledons does not take
place unless the mucosa is in contact with the
allantochorion. Contact with the trophoblast
alone is insufficient to induce change in the
stroma.
87. The placental barrier is of the syndesmochorial
type.
88. The uterine glands secrete intensively through-
:out pregnancy and become greatly dilated.
89. Until the 26th day the foetus is entirely
dependant upon the secretions of the uterine
epithelium and glands for its nourishment and it
is postulated that these secretions constitute
the principal source of nourishment of the
foetus throughout pregnancy.
90. Atrophy of the corpora lutea of pregnancy
commences at about the 18th week and the corpora
are markedly atretic at the time of parturition.
91. Follicular maturation is completely in abeyance
during pregnancy and the follicles which are
present at the time of conception atrophy.
Atrophy of the corpora lutea is followed by,
follicular growth but maturation and rupture of
follicles do not occur until the following
autumn.
92. It is concluded that follicular growth is
inhibited by a secretion of the corpus luteum.
93. During pregnancy the cervical canal becomes
distended by the accumulation within its lumen of
mucus secreted by its epithelium. Secretion of
mucus continues throughout pregnancy.
94. At parturition the mucus plug liquefies and great,
quantities of mucus flow into the vagina.
95. During pregnancy the superficial layers of the
vaginal epithelium tend to become converted into
cubical or columnar cells resembling in some
ways the mucus secretory cells of the cervical
epithelium, but mucus is never formed.
96. Desquamation ceases and keratinised cells are
never found in the vagina during the later stages,
of pregnancy. Leucocytic infiltration is
reduced to the anoestrous level.
97. The changes in the uterus, cervix and vagina
during the dioestrous cycle are compared with
those which occur during pregnancy: the
functional significance and the relations of the
changes to ovarian secretions are discussed.
The changes are compared also, to those which
have been described in other mammals.
98. The hypertrophic changes in the epithelia of the
uterine mucosa during int ero estrum are regarded
as analogous to those which occur during pregnancy,
and are throught to be due to the presence of a
luteal hormone. They are thus considered to be
equivalent in physiological significance to
those which occur in the rodents during "pseudo-
:pregnancy".
99. The degenerative changes occurring in the uterus
during oestrum are considered to be equivalent
to both the pro- oestrous and the pseudopregnant
degenerative phases in the bitch, the two being
superimposed in the ewe by the rapid onset of
heat following degeneration of the corpora lutea
of the previous cycle.
100. The rudimentary mucus plug formed during inter - :oestrum is regarded as the physiological
analogue of the true mucus plug formed during
pregnancy. The secretion of mucus is thought to
occur under the influence of the ovarian follicular hormone.
101. The liquefaction of the cervical mucus which
occurs during oestrum is not regarded as
equivalent to that which occurs in parturition.
Liquefaction is thought to be a non - specific
effect due, probably, to vascular congestion of
the cervix.
102. The primary function of the cervical mucus is
thought to be the sealing of the cervix during
pregnancy.
103. Keratinisation and desquamation of the vaginal
epithelium are thought to be remote effects of
the ovarian follicular hormone and to be devoid
of functional significance.
104. Some possible bearings of this study upon the
problem of the factors controlling fertility
in the ewe are discussed. It is suggested that
increased fertility as a result of nutritive
"flushing" may be due to the prevention of death
and resorption before attachment of the membranes
to the uterine mucosa has taken place, of some
of the foeti in multiple pregnancies; and that
"flushing" may not influence the number of ova
shed during oestrum
