262 



THE CELL 



one makes its way in from the surface, whereas just the reverse 

 occurs in Echinoderm eggs ; further, both nuclei are approxi- 

 mately of the same size, and lie close together, although for a 

 time they do not coalesce, but pass through a period of rest. 

 Indeed, even after they have begun to prepare for the formation 

 of the first division spindle, they do not commence to fuse. In 

 consequence of this, and of the further circumstance, that in 

 Ascaris megalocephala during nuclear division there develop only a 

 few nuclear segments, which are of considerable size, and hence 

 are easy to count, van Beneden (VI. 4a, 4b) was able to supple- 



Fio. 142. (I-I1I). Three diagrams depicting the course of the processes of fertilisation 

 iii Ascaris megalocephala bivalens. 



ment our knowledge of the process of fertilisation by the following 

 fundamental discovery : 



During the preparation for the first division spindle, the nuclein 

 in the egg- and sperm-nuclei, whilst these are still separated from 

 one another, becomes transformed into a delicate thread which 

 spreads itself out in many coils in the nuclear space. Each 

 thread then divides into two twisted loops of equal size, the 

 nuclear segments (Fig. 142 II). On either side of the pair of 

 nuclei a centrosome makes its appearance ; however, up till now, 

 no one has been so fortunate as to observe whence these are 

 derived. The line of demarcation between the two nuclei and the 

 surrounding yolk now disappears. 



Between the two centrosomes (Fig. 142 III), which are sur- 

 rounded by a radiation sphere, spindle fibrils develop ; these are 

 at first faint, but later on are distinctly visible; they arrange 

 themselves about the four nuclear segments, which have been set 

 free by the breaking up of the nuclear vesicles, so that they rest 

 externally upon the middle of the spindle. 





