64 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



time there appear in this finely granular material a number of 

 nuclei of a rather peculiar character. 



They vary immensely in size from that of an ordinary 

 nucleus to a size greater than the largest blastoderm-cell. 



In PI. 3, fig. i, n, is shewn their distribution in this finely 

 granular matter and their variation in size. But whatever may 

 be their size, they always possess the same characteristic struc- 

 ture. This is shewn in PI. 3, figs. I and 2, ;/. 



They are rather irregular in shape, with a tendency when 

 small to be roundish, and are divided by a number of lines into 

 distinct areas, in each of which a nucleolus is to be seen. The 

 lines dividing them into these areas have a tendency (in the 

 smaller specimens) to radiate from the centre, as shewn in PI. 3, 

 fig. I. 



. These nuclei colour red with haematoxylin and carmine and 

 brown with osmic acid, while the nucleoli or granules contained 

 in the areas also colour very intensely with all the three above- 

 named reagents. 



With such a peculiar structure, in favourable specimens these 

 nuclei are very easily recognised, and their distribution can be 

 determined without difficulty. They are not present alone in 

 the finely granular yolk, but also in the coarsely granular yolk 

 adjoining it. They form very often a special row, sometimes 

 still more markedly than in PI. 3, fig. i, along the floor 

 of the segmentation cavity. They are not, however, found 

 alone in the yolk. All the blastoderm-cells in the earlier stages 

 possess precisely similar nuclei ! From the appearance of the 

 first nucleus in a segmentation-sphere till a comparatively late 

 period in development, every nucleus which can be distinctly 

 seen is found to be of this character. In PL 3, fig. 2, this is 

 very distinctly shewn. 



(i) We have, then, nuclei of this very peculiar character 

 scattered through the subgerminal granular matter, and also 

 universally present in the cells of the blastoderm. (2) These 

 nuclei are distributed in a special manner under the floor of 

 the segmentation cavity on which new cells are continually 

 appearing. Putting these two facts together, there would be 

 the strongest presumption that these nuclei do actually become 

 the nuclei of cells which enter the blastoderm, and such is 



