3g K. MITSÜKXJEI; 



that given in Figs. 8 and 8«, and Series IX, with that of EJasmo 

 branch eml^ryos diagrammatically represented in Woodcuts XII or 

 XIII, or whether we compare the course of events th;it take place 

 in the two groups before or after such a stnge is reached, the resem- 

 blances are very striking. It would be preposterous to think that 

 all these similarities are mere coincidences. They can only be inter- 

 preted as having a deep phylogenetic significance. We may, I think, 

 safely conclude that the course of erents described in tlie preceding pages 

 as taking place in the posterior portion of tlie einhrijonic area resulting in 

 the formation of the posterior part of tlie endnnjo in Chelonia is a repeti- 

 tion, in a rudimentarg form, of the process affectiiaj tlie homologous parts in. 

 Elasmobranchii. 



If this is really the case, and the homologies of parts are indeed 

 those indicated in the last few pages — and I see no way of escape 

 from tliis conclusion — some very iniportant and far-reaching results 

 follow : — 



1. It has been shown both in Elasmobranchii and Chelonia that 

 the blastopore consists at a certain stage (Woodcut X, C, and AYood- 

 cut XII and XIII) of tliree parts; (rt) the anterior dilated part, (/>) a 

 groove or streak leading backwards from it and ending in (c) the 

 posterior enlarged part — the yolk-blastopore. But embryologists will, 

 I think, willingly admit that the second of these three parts in 

 Chelonia has all the characteristics of tlie primitive streak or groove seen 

 in otlier Amniota — is in fact a primitive streak.'^ Hence it follows that 

 tlie primitive strcidh is geneticallij the blastopore lips or at least a part of 

 tltem. 



Sî'.DGWiCK ( 92, p. 565) says in regard to this point : — " Balfour 



* It would uot lie difficult to compare the prioiitive streak of Aves, for instance, with the 

 above structure in Chelonia and establisli their homology ; Ijut the resemblances are so obvious 

 that [ do not think it necessary to go into deüiils. 



