Landacre, F. L.

The Ohio Journal of Science

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The Ohio Naturalist,PUBLISHED BYThe Biological Club of the Ohio State University.Volume VIII. FEBRUARY, 1908. No. 4.TABLE OF CONTENTSLAND ACRE—The Epi-branchial plaeodes of Ameiurus 251SCHAFFNER—On the Origin of Polar Conjugation in the Angiosperms 2o5IIINE—Some Observations d neerning the Effects of Freezing on Insect Larvae 258HAM.MOND—The Embryology of Oxalis corniculata 261STERKI—Some Notes on I'hilomyeus 265HAMMOND—MCCKAY—Meetings of the Jiiological Club 207J. C. II.—Rook Review 269THE EPI-BRANCHIAL PLACODES OF AMEIURUS.*F. L. LANDACRE.The study of the epi-branchial placodes of Ameiurus wastaken up with the object of ascertaining to what extent theyenter into the composition of the cranial nerves in this groupof Teleosts.The catfishes with their enormously hypertrophied system ofgustatory nerves ought to have these placodes correspondinglyprominent during embryonic development if they are concernedin the origin of the communis portions of the cranial ganglia.Ameiurus has proven to be an unusually favorable type anda study of the development of its epi-branchial placodes suggeststhe idea that possibly the extremely divergent accounts given inthe literature of these structures may be due to the choice ofunfavorable types for study.The extent to which communis fibres enter into the composi-tion of the cranial nerves varies greatly even among the teleosts.There is probably a corresponding variation in the distinctnesswith which the placodes may be traced in the early developmentof those types in which the system is reduced. At any rate theyare far more prominent and easily followed in Ameiurus thanthey seem to be in any other form described.Aside from working out the details of their origin the follow-ing queries have been kept more or less constantly in mind:(1) Do the cells derived wholly or in part from the placodesreally become the communis ganglia of the adult?(2) Are the placodal portions of the cranial ganglia pure—i. e., are the communis ganglia derived exclusively from the* Read at the 1907 meeting of the Ohio Academy of Science.252 The Ohio Naturalist. [Vol. VIII, No. 4,placodes or they do contain cells derived from the neural crest orthe mesectoderm?(3) Are the placodes in their early growth and at the time oftheir origin proportionate in size to the communis ganglia of theadult?(4) Is there any evidence that the placodes arise in responseto a stimulus furnished by the neural crest portion of the cranialganglia, as has been stated, or is there any evidence that theymay be in part or wholly endodermic in origin?(5) Lastly, if they are of pure ectodermic origin do theygive rise to both general and special visceral fibres or only tospecial visceral fibres?A brief description of the origin of the placodes will-answerseveral of these queries.In Ameiurus the placodes appear serially from anterior toposterior on the hyoid and first four true gills, none beingdeveloped on the fifth.They arise in the ectoderm just dorsal and posterior to thegill slit at the time when the endodermic evagination comes intocontact with the ectoderm.Their presence is first indicated by a thickening of theectoderm; this is followed by a period of active mitosis in theectodermic thickening and a proliferation of cells into themesoderm of the corresponding gill bar: This proliferated masspushes its way posteriorally and dorsally and finally becomesdetached from the skin which later assumes its normal thickness.The proliferated masses especially in the IX and first twodivisions of the X nerves are quite definite in outline and stainmuch darker than either the ectoderm from which they arose orthe mesoderm. This difference in staining seems to be due to theamount and character of the intercellular substance. In the VIInerve the placode is so large and the proliferation of cells so rapidthat in its earlier stages at least it is not so definite in outline asthose of the IX and X, but in all cases from the time the pro-liferation begins the cell mass is easy to follow.The size of the mass proliferated is apparently proportionateto that of the communis ganglia of the adult being largest in theVII nerve and much smaller in the IX and first two diivsionsof the X.The placode of the fourth true gili combines so closely witha neural crest portion that I am unable to compare it with theothers.All these ganglia of placodal origin have been followed up toa stage where the roots and chief trunks of the nerves are formedand there can be no doubt except in the third division of the Xthat they give rise to the communis ganglia of the adult.Feb., 1908.] Epi-branchial Placodes of Ameiurus. 253As to whether these placodes may possibly be partly endo-dermic in origin or not can be answered in the negative withoutany hesitation. The relations in the VII are somewhat confusedowing to the rapid proliferation of cells which seems to push theganglion forward into contact with the endoderm but I believethis placode is also purely ectodermic.The important question to be determined here is of course,whether these placodal masses contain cells derived from theneural crest or from the mesectoderm.They certainly do not arise in Ameiurus in response tostimuli furnished by the contact of neural crest cells with theskin. There are no neural crest cells that can be distinguishedas such, that come into contact with the skin where the placodesarise except in the third division of the X nerve.If the placodes arise in response to any stimulus furnished bythe embryo, it is much more probable that it is the stimulus derivedfrom the contact of the endodermic evagination that forms thegill slit.The contact of this evagination which occurs even in the VIInerve is coincident with or closely associated with the appearanceof the placode and while there may be no causal relation it is astriking fact and may be fundamental.In the third division of the X nerve the neural crest masscomes into contact with the skin about the time the placodeappears and the relation of the neural crest cells and of theplacodal cells is so intricate that I have been unable as yet toseparate them. This seems to me, however, to call for a differ-ent interpretation from the one mentioned above.' While there are no definite masses of neural crest cells addedto the placodes except in the third division of the X nerve, it ismuch more difficult to be sure as to whether cells usually desig-nated as mesectoderm may be incorporated with those derivedfrom the placodes, particularly in the VII nerve.The visceral portions of the IX ganglia and of the first twodivisions of the X seem to be pure placodal ganglia.In the VII where the placode is large and the proliferation ofcells rapid the borders of the growing ganglion are not so definiteand there may be a small portion of the ganglion that comes fromthe mesectoderm and is not derievd from the placode. It must,however, be very small. So that we can say for this type thatthe communis ganglia of the IX and the first two divisions of theX are practically pure placodal ganglia, that there is more doubtabout the gerniculate ganglion, although even here the incor-porated portion must be very small; while in the third division ofthe X there is a large neural crest portion which combines withthe placodal portion so intimately that it is not possible todistinguish them.254 The Ohio Naturalist. [Vol. VIII, No. 4,As to whether the placodal portions of the communis gangliagive rise to special visceral fibres only, or to both special andgeneral visceral fibres there is less certainty. The facts on whichwe may hazard a conclusion are as follows:Every cranial nerve containing gustatory fibres comes froma ganglion which can be traced wholly or in part to an epibranchial placode. The VII, the cranial ganglion supplying byfar the largest number of gustatory fibres comes from the largestepibranchial placode.The X, the cranial ganglion supplying the largest generalvisceral component contains the largest neural crest element.The smallest general visceral component is apparently in theVII nerve and there is possibly a small neural crest element inthe gerniculate ganglion.The IX nerve is most interesting in this connection. I amunable to find any neural crest cells in the communis ganglion ofthe IX nerve and Professor Herrick describes it as supplyingtaste buds throughout its whole area of distribution and makesno mention of its supplying general mucuos surfaces.If this is true we have a pure special visceral or gustatorynerve coming from a pure placodal ganglion in the case of the IX,and a very reasonable deduction from this is, that the placodesgive rise to that portion of the communis ganglion which suppliesspecial visceral or gustatory fibres, the portion supplyinggeneral visceral fibres coming from the neural crest.The latter is homologous then in origin with the generalvisceral ganglia of the cord, but differs from them in that itdissociates itself from the general cutaneous cranial ganglia andfuses with that derived from the placode,The general cutaneous ganglia of the cranial nerves arequite distinct and there is even less excuse for confusing thecutaneous and visceral ganglion in the embryo than there is inthe adult.If the epibranchial placodes give rise to the gustatory por-tions of the communis ganglia, we must look on the gustatorynerves as comprising a special visceral system because they endin visceral centers in the brain, and not because of any similarityin origin of the general and special visceral ganglia or because ofthe method of distribution of the gustatory organs.The ganglia are in no immediate sense related to generalvisceral ganglia in mode of origin, and the gustatory organs arehighly specialised and may be mainly ectodermic in position sothat I think the term special communis ought to be applied withcentral brain connections always in mind as the justification forthe term and this seems to be generally done.This generalization in regard to the origin of the gustatoryfibres is too far reaching to be rendered safe by a study of oneFeb., 1908.] Polar Conjugation in the Angiosperms. 255type. Other teleosts in which the gustatory system is reduced ascompared with Ameiurus, as well as Cyclostomes, Elasmobranchsand Amphibia must be studied with this particular point in view.In other forms the neural crest element is apparently fairlyconstant in those nerves which contain gustatory fibres. Thesmall size of the special visceral component and of the placodesin these types may account for the idea that the placodes arise inresponse to the stimulus furnished by the contact of the neuralcrest with the skin. The catfishes, however, are unique in thesize of their placodes and of their special visceral component.And if the IX ganglion should prove not to be a pure placodalganglion and the IX nerve of the adult does supply generalvisceral surfaces, I still think the evidence very strong that theplacodal portions of these ganglia give rise to the special visceralor gustatory fibres.And the special visceral system bears the same relation to thegeneral visceral that the Acustico-lateralis system bears to thegeneral cutaneous.

The Epi-Branchial Placodes of Ameiurus

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The Epi-Branchial Placodes of Ameiurus

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