DEVELOPMENT OF ELASMOBRANCH FISHES. 29 



pointed out that not only were the sides of the furrow beaded, 

 but that there appeared in the protoplasm, close to the furrows, 

 peculiar vacuole-like cavities, precisely similar to the cavities 

 which were the cause of the headings of the furrows. 



The presence of these seems to shew that the molecular 

 cohesion of the protoplasm becomes, as compared with other 

 parts, much diminished in the region where a furrow is about 

 to appear, so that before the protoplasm finally gives way along 

 a particular hne to form a furrow, its cohesion is broken at 

 numerous points in this region, and thus a series of vacuole- 

 like spaces is formed. 



If this is the true explanation of the formation of these 

 spaces, their presence gives considerable support to the views 

 of Dr Kleinenberg upon the causes of segmentation, so clearly 

 and precisely stated in his monograph upon Hydra; and is 

 opposed to any view which regards the forces which come into 

 play during segmentation as resident in the nucleus. 



I have not observed the peculiar threads of protoplasm 

 which Oellacher^ describes as crossing the commencing seg- 

 mentation furrows. I have also failed to discover any signs of 

 a concentration of the yolk-spherules, round one or two cen- 

 tres, in the segmentation spheres, similar to that observed by 

 Oellacher in the segmenting eggs of Osseous Fish. The ap- 

 pearances observed by him are probably connected with the 

 behaviour of the nucleus during segmentation, and are related 

 to the curious bodies I have already described. 



With reference to the nuclei which Oellacher^ has described 

 as occurring in the eggs of Osseous Fish during segmentation, 

 there can, I think, be little doubt that they are identical with 

 the peculiar nuclei in the Elasmobranch eggs. 



He^ says : 



In an unsegmented germ there occurred at a certain point 



in the section a small aggregation of round bodies. I do not 



feel satisfied whether these aggregations represent one or more 

 nuclei. 



Fig. 29 shews such aggregation; by focusing at its optical sec- 

 tion eleven unequally large rounded bodies measuring from 0.004 



1 Loc. cit, 

 ^ Loc. cit. 

 ' Loc. cit. p. 410, 411, &c. 



