344 The Ohio Journal of Science [Vol. XVI, No. 8, 



There is another point of contact between the ectoderm and 

 the main ganghon of X in the post-branchial groove where 

 there is an appreciable thickening of the skin and a definite 

 enlargement of the ganglion, also some evidence of contribution 

 of ectodermal cells to the ganglion, but no separate epibranchial 

 division. 



Summary. 



1. The epibranchial placodes of Squalus Acanthias arise as 

 proliferations of the ectoderm about the middle and dorsal 

 region of the corresponding branchial clefts. 



2. Contribution of cells by the placodes to the visceral 

 ganglia is by proliferation and mesial migration, the cells 

 coming into contact with the caudal extremity of the cor- 

 responding ganglia. 



3. With but two exceptions, the placodes are easily dis- 

 tinguishable from the other ectodermal thickenings in the 

 same regions. 



4. The placodal cells, in the course of migration, undergo a 

 process of metamorphosis, during which the nuclei become 

 darker and more finely granular, and in Gang. IX, smaller in 

 size. In the older ganglia these migrating masses of placodal 

 cells are completely fused with the general visceral masses and 

 the cells of the two components are indistinguishable from 

 each other. 



5. There is a general similarity in behavior between the 

 placodal cells in the shark and those of other forms in which 

 this process has already been described. 



6. The order of maturity of the epibranchial ganglia is 

 from anterior to posterior, in progressive stages. 



LITERATURE CITED. 



Landacre, F. L. 1910. The origin of the Cranial Ganglia in Ameivirus. Jour, of 



Comp. Neurology, Vol. XX. No. 4, p. 309. 

 Landacre, F. L. 1912. The Epibranchial Plocodcs of Lcpidosteus osseus and 



their Relation to the Cerebral Ganglia, ibid. Vol. XXII, No. 1, p. 1. 

 Landacre, F. L., and McLellan, Marie. 1912. The Cerebral Ganglia of The Embryo 



of Rana pipiens, ibid. Vol. XXII, No. 5, p. 461. 



