254 DEVELOPMENT OF ELASMOBRANCH FISHES. 



The next stage is the most important in the whole history 

 of the formation of the layers. Not only does it serve to shew, 

 that the process by which the layers are formed in Elasmo- 

 branchs can easily be derived from a simple gastrula type like 

 that of Amphioxus, but it also serves as the key by which other 

 meroblastic types of development may be explained. At the 

 very commencement of this stage the embryonic swelling be- 

 comes more conspicuously visible than it was. It now projects 

 above the level of the yolk in the form of a rim. At one point, 

 which eventually forms the termination of the axis of the 

 embryo, this projection is at its greatest ; while on either side of 

 this it gradually diminishes and finally vanishes. This pro- 

 jection I propose calling, as in my preliminary paper 1 , the em- 

 bryonic rim. 



The segmentation cavity can still be seen from the surface, 

 and a marked increase in the size of the blastoderm may be 

 noticed. During the stage last described, the growth was but 

 very slight ; hence the rather sudden and rapid growth which 

 now takes place becomes striking. 



Longitudinal sections at this stage, as at the earlier stages, 

 are the most instructive. Such a section on the same scale as 

 PI. 7, fig. 4, is represented in PI. 7, fig. 5. It passes parallel 

 to the long axis of the embryo, through the point of greatest 

 development of the embryonic ring. 



The three fresh features of the most striking kind are (i) 

 the complete envelopment of the segmentation cavity within the 

 lower layer cells, (2) the formation of the embryonic rim, (3) the 

 increase in distance between the posterior end of the blastoderm 

 and the segmentation cavity. The segmentation cavity has by 

 no means relatively increased in size. The roof has precisely its 

 earlier constitution, being composed of an internal lining of 

 lower layer cells and an external one of epiblast. The thin 

 lining of lower layer cells is, in the course of mounting the 

 sections, very apt to fall off ; but I am absolutely satisfied that 

 it is never absent. 



The floor of the cavity has undergone an important change, 

 being now formed by a layer of cells instead of by the yolk. A 



1 Qy. Journal Microsc. Science, Oct. 1874. [This Edition, No. V.] 



