ON THE DEVELOPMENT OF THE ELASMOBRANCH FISHES. 327 



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. XIII, fig. 1, 

 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 XIII, fig. 2, this is very distinctly shown. 



(1) 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 be- 

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

 is actually the case. In my account of a segmentation I 

 have, indeed, already mentioned this, and I will return to it, 

 but before doing so will enter more fully into the distribution 

 of these nuclei in the yolk. 



They appear in small numbers around the blastoderm at 

 the close of segmentation, and round each one of them there 

 may at this time be seen in osmic acid specimens, and with 

 high powers, a fine network similar to but finer than that 

 represented in PI. XIII, fig. 2, n y. This network cannot, 

 as a general rule, be traced far into the yolk, but in some 

 exceptionally thin specimens it may be seen in any part of 

 the fine granular yolk around the blastoderm, the meshes of 

 the network being, however, considerably coarser between 

 than around the nuclei. This network may be seen in the 

 fine granular material around the germ till the latest period 

 of which I have yet cut sections of the blastoderm. In 

 the later specimens, indeed, it is very much more distinctly 

 seen than in the earlier, owing to the fact that in parts of the 

 blastoderm, especially under the embryo, the yolk-granules 

 have disappeared partly or entirely, leaving only this fine 

 network with the nuclei in it. 



A specimen of this kind is represented in PI. XIII, fig. 2, 

 n y, where the meshes of the network are seen to be finer 

 immediately around the nuclei, and coarser in the intervals. 

 The specimen further shows in the clearest manner that this 

 network is not divided into areas, each representing a cell 

 and each containing a nucleus. I do not know to what 



