The morphogenesis of the pharyngeal region is
believed to be involved in the occurrence of so-called
"Branchial Cysts", a condition affecting human and canine
adolescents of comparable age, and which is characterised
by the appearance of cystic lesions on the lateral aspects
of the upper portion of the neck. These lesions are
usually small in the human, while in the dog, due to the
presence of a hair coat, they remain undiscovered until
they have become large and pendulous. It is generally
believed that branchial cysts arise from remnants of the
branchial apparatus which are induced to proliferate by
certain hormonal combinations present in the young
individual. Investigators (Bhaskar and Bernier, 1959)»
who recently examined a large number of human lesions deny
a purely branchiogenic origin however, and postulate that
epithelial remnants of bronchial or, what is more likely,
parotid origin, after becoming trapped in developing lymph
nodes, produce this condition, which according to their
view should be renamed "Benign Lymphoepithelial Cysts".Investigations into the etiology of branchial cysts
in the dog, at present being carried out at the author's
home university (Ontario Veterinary College, University of
Toronto), became focused on the developmental processes of
the pharyngeal region in that animal, because experience
with the human condition suggested a developmental defect.
2
A search of the literature available to the author revealed
that the development of the canine pharyngeal region was
practically unknown, only some selected aspects such as the
early development of the thyroid and parathyroids having
been described by Godwin (193^S 1937 a) of the Kingsbury
school. It was uncovered further that the developmental
anatomy of the dog as a whole had scarcely been studied,
which is surprising in view of this animal's importance to
the veterinary surgeon and to the research worker.The present study, then, was undertaken in an attempt
to close the obvious gap in our knowledge of canine
embryogenesis, and to provide those interested in the
etiology of branchial cysts with what might be considered
a working basis for further clinical and embryological
examinations. It was found necessary to survey the entire
field of pharyngeal development, from the appearance of a
foregut to conditions found at full term, since confinement
of the field, even though it could have been explored to
much greater depth, would have proved useful neither to
those interested in canine embryology nor to those studying
branchial cysts. The former would find that the large gap
in the knowledge of this region still persisted and the
latter would not be able to select the particular
anatomical region or developmental stage which may interest
them for further scrutiny.The lack of previous work on canine development and
subsequent absence of known standards of developmental
reference made it necessary to devote much space to the
description and staging of the einbryological material prior
to sectioning. The hulk of the thesis however is devoted
to the description of the developmental processes observed
in the pharyngeal region, and minor points worthy of short
discussion are dealt with here, because it was thought that
due to the wide field covered in this study a separate
discussion would appear rather disjointed. Major
anatomical areas however have been extracted from this
section and are presented at the end in the form of
REGIONAL REVIEWS
