June, 1916] Epibranchial Placodes of Squalus A canthias 341 



The length of the first branchial ganglion of X, is 270 

 microns and the area of contact is 90 microns in length and 

 situated toward the posterior end of the ganglion. This gives a 

 ratio of 1-3 between the length of the contact area and the 

 total ganglion length (Table I). In the 18 mm. stage, the 

 length of the area of contact is 100 microns (Table II). 



The oval outline of the ganglion, as seen anterior to the 

 point of contact, persists posterior to the first section in which 

 contact is seen (Fig. 22), but finally becomes indented by con- 

 tact with the placodal mass so that the lateral curve is lost 

 (Fig. 23 and 24), but the boundary between the general visceral 

 and placodal components is quite distinct and persists through- 

 out the entire length of the contact. The contact occurs toward 

 the posterior end of the gill cleft and is directly mesial to the 

 external aperture of the cleft. On account of its proximity to 

 this structure, it is impossible to distinguish the placode from 

 the other ectodermal thickenings associated with the gill clefts. 



In Fig. 22, the cells with dark, granular nuclei lie in contact 

 with the placode and there are no mitotic figures, showing that 

 the processes of contribution and metamorphosis are slower in 

 this region and so, more nearly complete toward the anterior 

 end of the contact. This is true of all ganglia. In the more 

 posterior sections (Fig. 23 and 24), there is evidence of more 

 active proliferation, since there are large masses of undifferen- 

 tiated cells to be seen, which have probably become detached 

 en masse, lying near the ectoderm. The smaller size of the 

 nuclei of the placodal cells is not so marked as in Gang. IX. 



The second branchial ganglion of X presents a different 

 arrangement from the first, in that the point of contact is with 

 the entodermal evagination from the pharynx which enters into 

 the formation of the gill cleft, at the anterior end; toward the 

 posterior end, the contact is with the ectoderm at a point 

 dorso-mesial to the external aperture of the cleft. The branchial 

 ganglion is entirely free from the main ganglionic mass of X 

 several sections anterior to the anterior end of the gill cleft. 



The length of the ganglion is 130 microns and the length of 

 the contact area is 90 microns, giving a ratio of 1-1.46 between 

 the contact length and the total ganglion length. In the 18 mm. 

 larva, the length of the contact area is 90 microns, also, though 

 the length of the ganglion is not so great as in the older stage 

 (Tables I and II). 



