93 I^'. MITSUKURI ; 



that this conckision strengthens their position, but I venture to 

 think that their view is not justifiable for reasons which I shall try 

 to show in the sequel. 



The above discussion makes it clear that in the amphibian egg there 

 is present onhj the primarg yolk-iuass, homologous with that of the Elasiuo- 

 hranch egg hit verij much smaller in quantitij. No clue has, so far as I 

 am aware, been yet discovered within the amphibian egg itself as 

 to whether it (1) had once a large yolk-mass like that of the Elasmo- 

 branch egg and then lost it in the course of time, or (2) represents a 

 condition similar to that which the Elasmobraneh egg must have 

 been in before it acquired the large yolk-mass. Rabl ( 89) adopts 

 the former of these alternatives. The systematic position of the 

 Amphibia in the scale of the vertebrate phylum would incline one 

 to do likewise, but the peculiar nature of the Rusconian yolk-plug 

 referred to above ought to make us cautious in giving the question a 

 final decision. It is certainly a remarkable fact that there is a 

 greater and more precise agreement in this aspect of development 

 between Elasmobranchii and Chelonia than between Elasmobranchii 

 and Amphibia. 



Let us next cast a glance over — 



THE AVES. 



When we compare the primitive streak in Birds and Chelonia, 

 the first thing that strikes us is the great difference in the time of the 

 first appearance of tliis structure in the two groups. In Birds the 

 primitive streak is one of the first structures visible in the blastoderm, 

 while the structure in Chelonia which I have described in the present 

 article and called the primitive streak comes into existence only 

 after the head, the medullary canal, the amnion, etc, have made 



