238 DEVELOPMENT OF ELASMOBRANCH FISHES. 



Instead of being but indistinctly separated from the sur- 

 rounding yolk, the blastoderm has now very clearly defined 

 limits. 



This is an especially marked feature of preparations made 

 with osmic acid. In these there may frequently be seen a 

 deeply stained doubly contoured line, which forms the limit of 

 the yolk, where it surrounds the germinal disc. Lines of this 

 kind are often to be seen on the surface of the yolk, or even of 

 the blastoderm, but are probably to be regarded as products of 

 reagents, rather than as organised structures. The outline of 

 the germinal disc is well rounded, though it is occasionally 

 broken, from the presence of a larger cell in the act of being 

 formed from the yolk. 



It is not probable that any great importance is to be at- 

 tached to the comparative distinctness of the outline of the 

 germinal disc at this stage, which is in a great measure due 

 to a cessation in the formation of fresh cells in the surrounding 

 yolk, and in part to the small and comparatively uniform size of 

 the cells of the germinal disc. 



The formation of fresh cells from the yolk nearly comes to 

 an end during this period, but it still continues on a small scale. 



The number of the nuclei around the germinal disc has 

 increased. 



Another feature of interest which first becomes apparent 

 during this stage is the asymmetry of the germinal disc. If a 

 section were made through the germinal disc, as it lay in situ in 

 the egg capsule, parallel or nearly so to the long axis of the 

 capsule, one end of the section would be found to be much 

 thicker than the other. There would in fact be a far larger 

 collection of cells at one extremity of the germinal disc than at 

 the other. The end at which this collection of cells is formed 

 points towards the end of the egg capsule opposite to that near 

 which the yolk is situated. This collection of cells is the first 

 trace of the embryo ; and with its appearance the segmentation 

 may be supposed to terminate. 



The section I have represented, though not quite parallel 

 to the long axis of the egg, is sufficiently nearly so to shew 

 the greater mass of cells at the embryonic end of the germinal 

 disc. 



