ELISHA MITCHEI^Iv SCIENTIFIC SOCIETY. 73 



vation of abnormalities resembling in a measure the 

 sickle, incline me to accept Duval's view of this struc- 

 ture, and with him to regard it as an inconstant feature 

 of no morphological importance. 



Hertwig, in his paper on " Urmundraiid Spina bifi- 

 da,'' (1892), touches on the question of meroblastic gas- 

 trulation, and it would seem that he no longer believes 

 in the existence of Roller's sickle. For in his brief 

 sketch of the manner in which the primitive streak is 

 formed, he follows Duval, and represents the streak as 

 arising by the coalescence the blastoderm edges. He 

 therefore comes to regard the edge of the young blas- 

 toderm as the blastopore. 



Hertwig does not look on the entire edge of the 

 young blastoderm as the blastopore, but for some 

 reason unknown to me divides it into a blastoporic part 

 and a part designated as the Uinvjachsungsrand , hy 

 which name he formerly (text-book) meant the entire 

 blastoderm edge. The edge of the teleost blastoderm 

 is likewise divided into blastopore and iimzjachsiDigs- 

 raiid. This division is surprising, for round the entire 

 edge of the teleost blastoderm there is an ingrowth of 

 cells, just as there is round the blastopore lip of the 

 amphibian embryo. And the existense of such an in- 

 growth is undoubtedly a ver}^ strong argument for re- 

 garding the whole edge as the blastopore* It would 

 be interesting to learn the facts that have induced Pro- 

 fessor Hertwig to divide the edge of the teleost blasto- 

 derm in this manner. 



But if Hertwig has come to regard the edge of the 

 blastoderm, or any part of it, as representing the ur- 

 miDidraiid in the bird embr3^o, it would seem that he 

 must have abandoned his former views on eastrulation 



