June, 1916] Epibranchial Placodes of Squal us Acanthias 337 



appear in other forms, as do the branchial clefts, in serial order 

 from anterior to posterior, and it seemed probable that the 

 various stages in development of the placodes of the VII, IX 

 and of the four divisions of the X nerve would present the same 

 kind of evidence that could be secured by a study of one placode, 

 such, for instance, as that of the IX nerve, through a series of 

 embryos of successively older stages. 



In determining the relation of a visceral ganglion to the 

 ectoderm, it is necessary to distinguish carefully, the following 

 ectodermal thickenings: fa) the lateral line placode; (b) the 

 thickening of the ectoderm at the point where the entodermal, 

 pharyngeal pocket joins the ectoderm; (c) the ectodermal 

 thickening extending anterior and posterior to (b) ; (d) thick- 

 enings of the epithelium of the entodermal, pharyngeal pocket 

 which, after the gill slits are open, are continuous with the 

 corresponding ectodermal thickenings (b). 



A comparison of the 20 mm. and 18 mm. stages indicates 

 that, of the ganglia in question, the VII undergoes the earliest 

 development, hence, in the later stages, presents a more highly 

 developed condition, and here, the placode is very m.arked and 

 easily distinguishable from the thickenings associated with the 

 gill clefts. 



The placodes are characterized by a thickening of the 

 ectoderm, by irregular arrangement of the cells, and by the 

 presence of numerous mitotic figures, indicating rapid cell 

 proliferation. 



In the cases where actual cell contribution is observed, this 

 activity takes place almost uniformly by migration of the cells 

 from the placode toward the ganglion, followed by metamor- 

 phosis as follows ; the cells on the mesial border of the migrating 

 mass exhibit the characteristic darkly-staining, granular nuclei 

 described by Landacre in Lepidosteus; in some cases, also, 

 smaller size of the nuclei than those of the placodal cells that 

 have not migrated, or of the cells of the visceral components, 

 though this is by no means as constant as in Lepidosteus, and 

 in fact, was not clearly marked in any case except Gang. IX. 



Table I shows the ratio between the length of the area of 

 contact between the placodes and the ganglia, and the total 

 length of the ganglia themselves. It will be seen from this 

 that the lengths of the areas of contact increase as we pass 



