]^02 ^^- ^IITSUKURI; 



they are already Avidely understood and -^vill make the differences 

 which I wish to bring out easily comprehensible. 



Diagrams I-IV represent the course of development in Elasmo- 

 branchii, and Y-IX the same in Chelonia. 



Diagrams I-III are exactly like A and B in Fig. 114, and A in 

 Fig. 115, of Hertwig's Lehrbuch (p. 135 or '92, Taf XX, Figs. 1—3.) 

 so that they hardly need any explanation. The blastoderm limited 

 by 1-1 in I has spread to 2-2 in II and to 4-4 in III. The 

 embryonic rim grows in length with the growth of the blastoderm.* 

 The edge of the blastoderm opposite to that on whicli the 

 embryonic rim is placed grows much faster than the latter : hence 

 the lines 1-1, ''2-''2, etc., are oblique and the exposed surface of 

 the yolk lies in the diagrams on the right of the sphere instead 

 of being at the pole opposite to that which the blastoderm occupies 

 in I. In IV and Wa, the peculitir mode in which tlie blastoderm 

 finally encloses the yolk is sliown. After the tail knob has arisen, that 

 part of the blastoderm edge nearest the tail-knob on each side comes 

 to meet the corresponding part of the opposite side ventral to, and 

 behind, the tail, thus removing the exposed surface of the yolk-mass 

 some distance behind the posterior end of the embryo. The blastopore 

 then consists of three parts, (1) the neurenteric canal (X curent C, TV 

 and IVrt), (2) the yolk-blastopore, and (3) the 2-shaped streak stretch- 

 ing between (1) and (2) and going round the posterior tip of the tail. 

 The yolk exposed during all these phases, is tlie priuiari/ noUi-iiiass. 

 There are reasons which make me think that the whole blastoderm 

 edge in Elasmobranchii re[)resents the blastopore lips and that the 

 division of it into two parts, the blastop(3re edge (Urmundrand) and 



* This much is admitted by all, as woU by those who uphold the concrescence theory as by 

 those who oppose that theory. 



