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



The distinction between the placode and the gill cleft 

 thickenings is difficult to determine, at least, in the more 

 anterior sections, though the irregular arrangement of the cells 

 of the ectoderm and the presence of a few mitotic figures are 

 evidences of proliferation. In the first section in which contact 

 is seen, the mass of contributed cells is quite large and persists 

 posterior to the point of disappearance of the visceral portion, 

 so that the placodal portion lies well toward the posterior end 

 of the ganglion, most of which lies anterior to the middle of 

 the gill cleft. 



The oval outline of the visceral component persists after 

 contact (Fig. 30) and the boundary between this and the pla- 

 codal component is quite distinct, even posterior to the point 

 at which the lateral curve of the visceral mass becomes indented 

 by contact with the placodal mass (Fig. 31). In the more 

 posterior sections (Fig. 32 and 33), there is fusion between the 

 two components to such an extent that the boundary is not so 

 distinct. There is evidence of rapid contribution in the pres- 

 ence of a large mass of undifferentiated cells near the ectoderm 

 (Figs. 30, 31 and 32). The metamorphosing cells possess nuclei 

 but slightly smaller in size than those of the other cells of the 

 ganglion. 



In the third branchial ganglion of X, also, the most anterior 

 contact is with the entodermal gill pocket from the pharynx 

 instead of with the ectoderm (Fig. 34, 35, 36 and 37). The 

 length of the ganglion is 150 microns and that of the contact 

 area, 120 microns, giving a ratio of 1-1.25 between the length 

 of the area of contact and that of the total ganglion. In the 

 18 mm. larva, the length of the contact area is 80 microns, 

 showing that the process of contribution has probably not 

 progressed to so great a degree in the 20 mm. stage as it has in 

 the VII and IX ganglia in the younger stage (Tables I and II). 



In the more anterior regions, it is impossible to distinguish 

 between the placode and the gill cleft thickenings but, in the 

 more posterior regions, the distinction is quite clear (Fig. 39, 

 40 and 41). Proliferation and metamorphosis are evidently 

 going on quite rapidly throughout the entire length of the 

 contact and the placodal mass persists in considerable size to 

 the posterior end of the ganglion. There is no appreciable 

 difference in size between the nuclei of the metamorphosing 

 cells and those of the neighboring cells but the dark stain and 



