DEVELOPMENT OF ELASMOBRANCH FISHES. 41 



branchs can easily be derived from a simple gastrula type like 

 that of Ampbioxus, 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 

 becomes more conspicuously visible than it was. It now pro- 

 jects 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 ; w^hile on either side 

 of this it gradually diminishes and finally vanishes. This pro- 

 jection I propose calling, as in my preliminary paper^ 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 

 PL III. fig. 4, is represented in PL iii. fig. 5. It passes parallel 

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

 development of the embryonic rim. 



The three fresh features of the most striking kind are (1) 

 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 abso- 

 lutely 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 

 precisely similar but more partial change in the constitution 

 of the floor takes place in Osseous Fishes ^ 



1 Qy. Journal Microsc. Science, Oct. 1874. 



- Gotte, Der Keim cl. Forelleneies, Arch. f. Mikr. Anat. Vol. is.; Haeckel, 

 Die Gastrula u. die Eifurcliung d. Thiere, Jenaische Zeitschrift, Bd.ix. 



