58 



MEMOIRS OF THE NEW YORK ACADEMY OF SCIENCES 



mechanical cause no longer exists. But I believe that the homology proposed by 

 DEAN is well founded. Through Professor DEAN'S kindness, I have had the privilege 

 of studying some fine Amia material, and have found therein the counterpart of the 

 phenomena just described for the teleosts. 



Figure 16 shows a longitudinal (nearly or quite sagittal) section through the 

 egg of Amia at a time when the yolk-cells are perhaps three-quarters covered. The 

 superficial layer of the epiblast is seen to be much thicker at the blastopore margin 

 than elsewhere and to be directly continuous, around the margin, with the inner- 

 most germ-layer. This condition occurs upon the entire periphery, but on the dor- 

 sal (embryonic) side of the blastopore the case is somewhat more complicated. 

 Here we find an arrangement strictly comparable to that found in figure 13. In 

 both there is an enormous development of the pavement layer at the caudal end of 



the embryo and an indentation of its 



FIGURE 17. 



p Q 



N 6 H V 



1 *. 



Sy GHy'" *^ 



Blastopore region of Amia after closure (drawn as preceding). 

 Up, blastopore; Ne Hy, "non-embryonic" hypoblast; GHy, 

 gut-liypoblast ; Kv, probable homologue of Kupffer's Vesicle ; 

 Sy, rudimentary syncytium. 



surface around which the cells are 

 arranged in radial manner. In one 



O 



respect, hoAvever, this Amia embryo 

 exhibits a more advanced condition 

 than that of the Murasnaf embryo 

 shown in figure 13. The notch in 

 the pavement layer is seen to be con- 

 tinued cephalad into a very notice- 

 able cleft, extending forward for some 

 distance into the hypoblast and sep- 

 arating it into two layers. 



"Again the similarity between figures 17 and 14 is evident. The chief difference 

 between the two seems to be that in the former there is wanting the open canal, con- 

 necting the cavity of the vesicle with the exterior which occurs in the latter. This 

 open canal is wanting even in embryos slightly younger than that shown in figure 

 17, but the obliteration of its lumen is surely a matter of secondary importance. 

 The ganoidean homologue of Kupffer's Vesicle, like that of many teleosts, has 

 a ventral wall. This is well developed, though not quite complete, consisting of a 

 layer of pavement-like cells, lying directly upon the large yolk cells. This condi- 

 tion has not been mentioned by either DEAN ('96) or SOBOTTA ('96). 



