DEVELOPMENT OF ELASMOEllA^'CH FISHES. 47 



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 PI. IV. 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 chanofes 

 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 PL iv. fig. 8 a, 8 b, 

 and 8 c. 



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 

 the rest of the germ, but by the conversion of the lower layer 

 cells into hypoblast having been carried far further towards the 

 centre of the oferm in the axial line than in the lateral res^ions 

 of the rim. 



The most anterior of the series of transverse sections (PI. iv. 

 fig. 8c) I have represented, is especially instructive with reference 



