24 SEGMENTATION. 



bodies afford a strong presumptive evidence of a change in the 

 manner of nuclear increase. 



The hist argument I propose urging on this head is derived 

 from the bodies (PI. ii. fig. 8 a, h, c) which I hav« described as 

 intermediate between the true cone-like bodies and typical 

 nuclei. They appear to afford evidence of less and less of the 

 matter of the nucleus streaming out into the yolk and of a 

 large proportion of it becoming divided. 



The conclusion to be derived from all these facts is that 

 for Elasmobranchs in the earlier stages of segmentation, and 

 during the formation of fresh segments, a partial solution of 

 the old nucleus takes place, but all its constituents serve for 

 the reconstruction of the fresh nuclei. 



In later periods of development a still smaller part of the 

 nucleus becomes dissolved, and the rest divides; but the two 

 fresh nuclei are still derived from the two sources. After the 

 close of segmentation the fresh nuclei are formed by a simple 

 division of the older ones. 



The appearance of the cone-like bodies in the yolk outside 

 the germinal disc is a point of some interest. It demonstrates 

 in a conclusive manner that whatever influence (if any) the 

 nucleus may have in ordinary cases of cell division, yet it may 

 undergo changes of a precisely similar character to those which 

 it experiences during cell division, without exerting any influence 

 on the surrounding protoplasm \ If the lobate nuclei are also 

 nuclei undergoing division, we have in the Qgg of an Elasmo- 

 branch examples of all the known forms of nuclear increase 

 unaccompanied by cell division. 



The next stage in the segmentation does not present so 

 many features of interest as the last one. The segments are 

 now so small, as to be barely visible from the surface with a 

 simple lens. A section of an embryo of this stage is repre- 

 sented in PI. II. fig. 8. The section, which is drawn on the 



1 Strasbiirger's [loc. cit.) arguments about the influence of tbe nucleus in 

 cell tlivision are not to my mind conclusive; though not without importance. 

 It is diiUcult to reconcile his views with the facts of cell division observable 

 during the Elasmobranch segmentation; but even if their truth be admitted they 

 do not bring us much nearer to a satisfactory understanding of cell division, un- 

 lei^s accompinied (and at present they are not so) by a rational explanation of 

 the forces which produce the division of the nucleus. 



