DEVELOPMENT OF THE ELASMOBRANCH FISHES. 63 



this particular the segmentation in these animals differs materi- 

 ally from that in the Bird, where the horizontal furrows appear 

 very early. 



In each segment a nucleus was generally to be seen in sec- 

 tions. I will, however, reserve my remarks upon the nature of 

 the nuclei till I discuss the nuclei of the blastoderm as a whole. 



For some little time the peripheral segments continue larger 

 than the more central ones, but this difference of size becomes 

 less and less marked, and before the segments have become too 

 small to be seen with the simple microscope, their size appears 

 to be uniform over the whole surface of the blastoderm. 



In the blastoderms somewhat older than the one last de- 

 scribed the segments have already become completely separate 

 masses, and each of them already possesses a distinct nucleus. 

 They form a layer one or two segments deep. The limits of the 

 blastoderm are not, however, defined by the already completed 

 segments, but outside these new segments continue to be formed 

 around nuclei which appear in the yolk. At this stage there is, 

 therefore, no line of demarcation between the germ and the yolk, 

 but the yolk is being bored into, so to speak, by a continuous 

 process of fresh segmentation. 



The further segmentation of the already existing spheres, 

 and the formation of new ones from the yolk below and to the 

 sides, continues till the central cells acquire their final size, the 

 peripheral ones being still large, and undefined towards the yolk. 

 These also soon reach the final size, and the blastoderm then 

 becomes rounded off towards the yolk and sharply separated 

 from it. 



The Nuclei of the Yolk. 



Intimately connected with the segmentation is the appear- 

 ance and history of a number of nuclei which arise in the yolk 

 surrounding the blastoderm 



When the horizontal furrows appear which first separate the 

 blastoderm from the yolk, the separation does not occur along 

 the line of passage from the fine to the coarse yolk, but in the 

 former at some distance from this line. 



The blastoderm thus rests upon a mass of finely granular 

 material, from which, however, it is sharply separated. At this 



