FEMALE REPRODUCTIVE CELLS 465 



cular pressure or the crowding of the ova behind it, to be passed out 

 of the animal's body into the surrounding water, where there is a large 

 probability of its being near the spermatozoa which have been similarly 

 deposited by neighboring males. 



The sea water influences the ovum to go through the maturation 

 divisions. The nucleolus and the chromosome groups are usually to 

 be found at this time nearest the distal edge of the eccentric nucleus. 

 In the most ordinary cases the nucleolus begins to show an irregular 

 outline, the distal part of the nuclear membrane begins to undulate 

 and shrivel, and a radiating figure of achromatic material, the aster, 

 appears in the cytoplasm just distal of the nucleus (Fig. 432). In fact, 

 it is the astral rays, combined probably with some solvent agency, 

 which seem to cause the nuclear membrane to give before them. Pres- 

 ently it can be seen that the many chromosomes have separated some- 

 what and moved up to the aster. This latter has divided and sepa- 

 rated, leaving a set of connecting fibrils, the spindle fibrils, which extend 

 between the daughter asters. 



As the chromosomes move up into the area between the separated 

 asters, the nucleolus begins to discharge its chromatin as a se'ries of 

 irregular, semi-fluid lumps which leave behind them the plastin body 

 that held them during the growth period (see Fig. 432). This plastin 

 remnant does not stain deeply, and after becoming irregular it breaks 

 up and disappears. Meanwhile, the chromatin which left it is separated 

 into smaller granules, and part of these appear to act as a source of nutri- 

 ment to the chromosomes, which increase considerably in size at this 

 time. Part of this chromatin is distributed through the rest of the 

 nucleus, which has now lost its membrane and appears as a larger, more 

 granular and darker-staining area, the " residual substance," in the 

 cytoplasm. 



Often the passing of chromatic material from nucleolus to chromo- 

 somes begins to take place before the other maturation phenomena 

 have begun. This is most apt to be seen in cases where the chromo- 

 some masses and nucleolus are at some distance from the point at which 

 the spindle is formed. Figure 433 shows such a case and also shows 

 how the chromatin passes out of the nucleolus in a fine, granular stream. 

 Part of the plastin ground substance has been left free by the chromatin 

 in this instance. 



Shortly after such a figure as 432 the spindle is formed, and the chro- 

 mosomes, at first widely scattered, begin to be drawn into a fairly regular 

 equatorial plate (Fig. 434, A). They have already begun to divide by 

 a longitudinal division before the plate is actually formed. Shortly after 

 this the figure appears as in Figure 434, B, where some of the chromo- 

 somes have already divided, and the others are soon to follow. They 



