No. 2.] THE DEVELOPMENT OF THE NEWT. 347 



line. Any one portion of this fused area therefore is just 

 as much entitled to be considered primitive streak as any 

 other. 



I must dissent, however, from the singular conclusion of 

 Robinson and Assheton that the primitive streak of Bonibinator, 

 Triton and Petromyzon is not homologous with the primitive 

 streak of Rana temporaria. I do not think that variations in 

 direction of formation or in the relative position of the anus 

 are sufficient reasons for the rejection of the homology of this 

 fused linear mass of germ layers. As I have endeavored to 

 show, the method of closure of the blastopore and the position 

 of the anus are not constant, even in the same group of 

 animals, and hence cannot be looked upon as dependent on 

 purely primary causes. 



I consider, therefore, that the primitive streak, wherever 

 found, is the expression of the fusion of the blastopore lips. 

 (Prostomanaht). That in different groups of animals the 

 primitive streak should vary widely in character and extent 

 is not surprising, 



VII. Germ-layers and Notochord. 



Since I believe that the prevailing uncertainty regarding the 

 origin and extent of the germ-layers in amphibia can be suc- 

 cessfully met only by a comparative study, I shall not attempt 

 at this time any extensive discussion of my results upon the 

 newt. It does not seem to me likely that the events in a single 

 animal, however illuminating in themselves, can be regarded as 

 "characteristic" or "typical" of the whole group. For this 

 reason I shall for the present simply describe the early develop- 

 ment of the germ-layers in the newt and shall draw none but 

 the most obvious inferences from my work. 



The ectoblast in the newt is derived chiefly from the small 

 cells of the upper pole, although cells split off from the upper 

 end of the yolk-cells are at first constantly add^d to these. 

 There is always a certain area of yolk-cells — the yolk-plug — 

 which, although originally superficial, is ultimately covered in 

 by the downward growth of small ectoblast cells. I have else- 



