260 DEVELOPMENT OF ELASMOBRANCH FISHES. 



of the primitive alimentary involution in all anamniotic verte- 

 brates with the exception of Amphioxus. 



Leaving the segmentation cavity I pass on to the other 

 features of my sections. 



There is still to be seen a considerable aggregation of cells 

 at the non-embryonic end of the blastoderm. The position of 

 this, and its relations with the portion of the blastoderm which 

 at an earlier period contained the segmentation cavity, indicate 

 that the growth of the blastoderm is not confined to its edge, 

 but that it proceeds at all points causing the peripheral parts to 

 glide over the yolk. 



The main features of the cells of this blastoderm are the 

 same as they were in the one last described. In the non- 

 embryonic region the epiblast has thinned out, and is composed 

 of a single row of cells, which, in the succeeding stages, become 

 much flattened. 



The lower layer cells over the greater part of their extent, 

 have not undergone any histological changes of importance. 

 Amongst them may frequently be seen a few exceptionally 

 large cells, which without doubt have been derived directly 

 from the yolk. 



The embryonic rim is now a far more considerable structure 

 than it was. Vide PL 7, fig. 7. its elongation is mainly effected 

 by the continuous conversion of rounded lower layer cells into 

 columnar hypoblast cells at its central or anterior extremity. 



This conversion of the lower layer cells into hypoblast cells 

 is still easy to follow, and in every section cells intermediate 

 between the two are to be seen. The nature of the changes 

 which are taking place requires for its elucidation transverse as 

 well as longitudinal sections. Transverse sections of a slightly 

 older embryo than B are represented on PI. 7, fig. 8 a, 81? 

 and 8c. 



Of these sections a is the most peripheral or posterior, and c 

 the most central or anterior. By a combination of transverse 

 and longitudinal sections, and by an inspection of a surface view, 

 it is rendered clear that, though the embryonic rim is a far more 

 considerable structure in the region of the embryo than else- 

 where (compare fig. 6 and fig. 7 and 7 ' a\ yet that this gain in 

 size is not produced by an outgrowth of the embryo beyond 



