1 8 JE'NKI'NSO'N, Gervimal Laj^ers of Vertebrates. 



Bdellostovia. 



Our knowledge of the early phases of development in 

 the megalecithal and meroblastic Myxinoids is exceed- 

 ingly meagre, but Bashford Dean has shown that in 



Fig. 6. 

 Baellostoma (after Dean). 



A. Blastoderm at one pole of the ellipsoid egg ; op.r., opercular ring 

 of the shell. 



B. Dorsal lip (</./.) beginning to grow down. 



C. A little later : v.l, ventral lip. 



D. Embryo formed : blastopore closing at vegetative pole. 



Bdellostoma the ^^^ axis is identical with the major axis 

 of the ellipsoid &%%, and that segmentation produces a cap 

 of cells or blastoderm'' at one pole i^Fig. 6, A). At one 

 point in the edge of this blastoderm a dorsal blastoporic 

 lip appears, and the material for the germ-layers of the 

 embryo is laid down during the bilateral overgrowth and 



^ Throughout this paper I have followed the customary usage in 

 employing one term, " blastoderm," to denote the cap of cells produced by 

 segmentation at the animal pole of large-yolked eggs, both in the Anamnia 

 and Amniotes ; and having regard to the similarity in origin of the cap of 

 cells in question in the two cases the use of a common term is justifiable. 

 In their subsequent history, however, as will be fully shown below, the 

 Anamnian and the Amniote "blastoderms" differ widely; and it might 

 therefore be advisable to retain the expression for one of these only if a 

 suitable word could be found for the other. 



