June, 1916] Epibranchial Placodes of Squalus Acanthias 339 



Gang. VII. 



Since this ganglion is the earliest to develop, there is, in 

 the 20 mm. stage, a relatively small number of cells showing the 

 characteristic distinguishing features of placodal cells. The 

 ganglion is 340 microns long, but for only 10 microns or through 

 one section, was there any actual contact with the ectoderm, 

 while in the 18 mm. stage there is contact through 18 sections 

 or 180 microns (Tables I and 11). In the 22 mm. stage, there 

 is no contact and no cells which show the characteristic nuclei 

 to a degree sufficient to permit of their being distinguished from 

 the general visceral cells. 



In the 20 mm. embryo, most of the ganglion lies anterior and 

 dorso-mesial to the anterior extremity of the first true gill cleft, 

 and the point of contact is just opposite the anterior end of the 

 cleft. There are a number of mitotic figures in the placode and 

 the migrating cell mass, showing that the processes of prolifera- 

 tion and metamorphosis are not yet com.pleted. 



The placode is an extensive thickening of the skin and at 

 one point (Fig. 1 and 8), there seems to be a tendency to- 

 lamellation. The placode lies quite free and distinct from the 

 thickening of the ectoderm which accompanies the opening- 

 of the gill cleft (c). Throughout most of its extent, the ganglion 

 shows a well defined, encapsulated outline, which condition 

 indicates maturity since it is not present to so great a degree in 

 the more posterior ganglia. 



The ventro-lateral lateralis component (Fig. 1 to 9, V. L. 

 VII), constitutes the dorsal portion of the ganglionic mass and 

 extends several sections posterior to the limits of the visceral 

 portion. 



Gang. IX. 



This ganglion is 190 microns in length and the contact area 

 occupies 40 microns of this length, giving a ratio of 1-4.75 

 between the total length and the length of the area of contact. 

 The point of contact is approximately opposite the middle of 

 the gill cleft and toward the posterior end of the ganglion. In 

 the 18 mm. stage, the contact area is 160 microns long and so 

 is not much smaller in extent than the contact area of VII in. 

 the same stage (Table II). 



