62 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



lar disc, distinctly marked off by a dark line from the rest of the 

 yolk. This line, as is proved by sections, is the indication of a 

 very shallow groove. The appearance of sharpness of distinc- 

 tion between the germ and the yolk is further intensified by 

 their marked difference of colour, the germ itself being usually 

 of a darker shade than the remainder of the yolk ; while around 

 its edge, and apparently sharply separated from it by the groove 

 before mentioned, is a ring of a different shade which graduates 

 at its outer border into the normal shade of the yolk. 



These appearances are proved by transverse sections to be 

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

 separating the blastoderm from the yolk. In the passage be- 

 tween the fine granular matter of the germ to the coarser yolk- 

 spheres every intermediate size of granule is present; and, 

 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 which 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 blasto- 

 derm was not defined by a true furrow, and the segmentation 

 furrows still ended below without meeting any horizontal fur- 

 rows, 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 



