ON THE DEVELOPMENT OF THE ELASMOBRANCH FISHES. 325 



deceptive. There is no sharp line either at the sides or be- 

 low separating the blastoderm from the yolk. In the passage 

 between the fine granular matter of the germ to the coarser 

 yolk-spheres every intermediate size of granule is present; 

 andj though the space between the two is rather narrow, in 

 no sense of the word can there be said to be any break or 

 line between them. 



This gradual passage stands in marked contrast with what 

 we shall find to be the case at the close of the segmentation. 

 In the youngest egg Avhich I had, the germinal disc was 

 already divided into four segments by two furrows at right 

 angles. These furrows, however, did not reach its edge ; 

 and from my sections I have found that they were not 

 cut off below by any horizontal furrow. So that the four 

 segments were continuous below with the remainder of the 

 germ without a break. 



In the next youngest specimen which I had, there were 

 already present eighteen segments, somewhat irregular in size, 

 but which might roughly be divided into an outer ring of 

 larger spheres, separated, as it were, by a circular furrow 

 from an inner series of smaller segments. The furrows in 

 this case reached quite to the edge of the germinal disc. 



The remarks I made in reference to the earlier specimen 

 about the separation of the germ from the yolk apply in 

 every particular to the present one. The external limit 

 of the blastoderm was not defined by a true furrow, 

 and the segmentation furrows still ended below without 

 meeting any horizontal furrows, so that the blastoderm was 

 not yet separated by any line from the remainder of the yolk, 

 and the segments of which it was composed were still only 

 circumscribed upon five sides. In this particular the seg- 

 mentation in these animals differs materially from that in the 

 Bird, where the horizontal furrows appear very early. 



In each segment a nucleus was generally to be seen in 

 sections. I will, however, reserve my remarks upon the 

 nature of the nuclei till I discuss the nuclei of the blasto- 

 derm 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 blastoderms somewhat older than the one last described 

 the segments have already become completely separate masses, 

 and each of them already possesses a distinct nucleus. They 



