SEGMENTATION. 241 



of the sides of the earlier formed furrows are distinctly shewn. 

 No mention of these is made in the text, but they are un- 

 questionably similar to those I have described in the Elasmo- 

 branch furrows. In the case of Elasmobranchs I 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 line 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 1 describes as crossing the commencing segmentation 

 furrows. I have also failed to discover any signs of a concentra- 

 tion of the yolk-spherules, round one or two centres, in the 

 segmentation spheres, similar to that observed by Oellacher 

 in the segmenting eggs of Osseous Fish. The appearances 

 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' 2 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 3 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. 



1 Lot. cit. - Lot. cit. 3 Lot. tit. pp. 410, 411, &c. 



