92 K. MITSÜKURI; 



has long ago pointed out that " the possibiht}^ of a comparison 

 between the Sauropsida and Elasmobranchii depends upon ex- 

 planation being possible of (1) the position of the embryo near the 

 centre of tlie hlastoderni, and (2) the nature of the primitive streak." 

 (Comparative Embryology. 1st ed., vol. ii, ch. XI, p. 2o(3). The 

 second of these points has already been disposed of, as the primitive 

 streak has been shown to be identical with the blastopore lips. The 

 other point, one which, I tliink, has bothered embryologists a great deal 

 more than the second, is also cleared away. For, if the problematical 

 cell-mass which I have called the yolk-plug represents the exposed 

 surf ice of the primary yolk-mass, then the emhrijo is actualhj formed at 

 the edge of tliis iiolk-inass^ and. not in the centre of the hîastodenn. Thus 

 a strict comparison n-itli the Elasmobranchii in this respect is possible. 

 Let us next turn our attention to — 



THE AMPHIBIA. 



A flood of light has lately been thrown on the process of 

 gastrulation in Amphibia by a number of important works by Koux, 



GoTTE, MOEGAX, Y. ErLAXGER ('89), ROBIXSOX and ASSHETOX ('91), 0. 



Hertwig ('92 and '95), Eycleshymer ('95), and many others. 

 A ci3mparison of the two grou}:)s, Chelonia and Amphibia, in this 

 respect brings out some important points interesting to us. I shall 

 now touch on some of these points. 



1. It has lieen shown by 0. Hertwig that the tail in the 

 Amphibia is formed by the rising up of a special cell mass, and is 

 formed in a way different from the rest of the body lying anteriorly 

 to the tail ( 92, p. ^52 et. seq.). It, moreover, arises from a paired 

 " Anlage.'' This must he considered the same as the process of 

 upheaval which I have brouo-ht to liü-ht in this article in reo-ard to the 



