io6 



NUCLEI OF THE YOLK. 



of a fairly uniform size. So much may be observed in surface views of the 

 segmenting ovum, and it may be noted that there is not much difference to 

 be observed between the segmentation of the germinal disc of the Fowl's 

 ovum and that of the Elasmobranchii. Indeed the figure of the former (fig. 

 44) would serve fairly well for the latter. When however we examine 

 the segmenting germinal discs by means of sections, there are some dif- 

 ferences between the two types, and several interesting features which 

 deserve to be noticed in the segmentation of the Elasmobranchii. In the 

 first stages the furrows visible on the surface are merely furrows, which 

 do not meet so as to isolate distinct segments ; they merely, in fact, form a 

 surface pattern. It is not till after the appearance of the equatorial furrow 

 that the segments begin to be distinctly isolated. In the subsequent stages 

 not only do the segments already existing in the germinal disc increase by 

 division, but fresh segments are continually being formed from the adjacent 

 yolk, and added to those already present in the germinal disc. (Fig. 46.) 



FIG. 46. SECTION THROUGH GERMINAL DISC OF A PRISTIURUS EMBRYO DURING 



THE SEGMENTATION. 



n. nucleus ; nx . nucleus modified prior to division ; nx'. modified nucleus of the 

 yolk ; /. furrow appearing in the yolk adjacent to the germinal disc. 



This fact is one out of many which prove that the germinal disc is merely 

 part of the ovum characterized by the presence of more protoplasm than the 

 remainder which forms the so-called food-yolk. During the latest stages of 

 segmentation there appear in the yolk around the blastoderm a number of 

 nuclei. (Fig. 46, nx-'.} These are connected with a special protoplasmic 

 network (already described) which penetrates through the yolk. Towards 

 the end of segmentation, and during the early periods of development which 

 succeed the segmentation, these nuclei become very numerous. (Fig. 47 

 A, n'.~) Around many of them a protoplasmic investment is established, and 

 cells are thus formed which eventually enter the blastoderm. 



The result of segmentation is the formation of a lens-shaped mass of 

 cells lying in a depression on the yolk. In this a cavity appears, the 

 homologue of the segmentation cavity already spoken of. It lies at first in 



