FATE OF BLASTOPOKE, ETC., IX CHELOXIA. 79 



not go into this question further on the present occasion, as I am 

 engaged with certain experimental investigations on the subject, a 

 preHminary acc<)unt of which ought soon to appear in the Anatoiiiisch er 

 Anzeiger * 



III. THEORETICAL CONSIDERATIONS. 



I have thus far used the names, the priuitlice streak, llie priinitice 

 groove, tlie i/olk-plug, etc., more for the sake of brevity, and have not 

 yet examined in defai] the nature of the respective structures to which 

 these names have Ijeen applied in the foregoing pages. This I pi'opose 

 to do in this section by comj)aring the course of events, whicli J hope, 

 the present investigation has fuily established i'ov the grouj) of 

 Chelonia, with what takes place in other groups of Yertebrata. 



For this purpose, I take up first — 



THE ELASIYIOBRANCHII. 



Thanks to the recent works of Schwarz (*89), Sedgwick ('92) 

 and the Zieglers (*92), not to mention the older and classical 

 memoirs of Balfour and others, a great deal of light has been thrown 

 on the development of the posterior part of the embryo in this group. 

 As what takes place in the Elasmobranchs serves as a sort of key in 

 explaining the course of changes in Chelonia, it is necessary that the 

 salient points of development in the first group should be given here. 

 I quote freely from Sedgwick (p. 559 et seip^ : — 



" As is well known, the blastoderm attains a certain size before 

 any trace of the embryo is visible, spreading by a uniform growth at 



* since appeared in .■//(((f()//(/.<c/(t')- Anzeiger. Xr. 13, XI Bd. 



