42 



THE MATURATION OF THE EGG OF THE MOUSE. 



chance occurrences, it must necessarily follow that the first polar cell 



may, in many cases does, dwindle to almost nothing. Indeed, it may 



even disappear completely; for out of the 507 eggs with complete second 



spindle, 189 have no polar cell. This is made clearer still when the 200 



eggs, mentioned above, are examined further. The results are most 



conveniently presented in tabular form (table 5). This shows that of 



the older eggs, as compared with the younger ones, fewer have the large 



polar cells and more have no polar cell. The fewer cases with small 



polar cell among the older eggs show that most of the polar cells which 



degenerate do so early, being completely wanting in the later epochs. 



The same conclusions are borne out by the 162 eggs of Stages IX to 



XI (table 2, p. 14), which, as a whole, cover a longer period. Of these 



162 eggs, 77 have no polar cell, 22 have a small polar cell, and 63 the 



larger sizes of polar cell. 



Table 5. 



The first polar cell contains the peripheral group of chromosomes, 

 which have become compacted into a single, usually flattened mass 

 (plate 4, fig. 18). During the formation of the second spindle this mass 

 divides into irregular parts (fig. 19a), which remain more or less in conti- 

 nuity with one another. It is only rarely that these parts separate from 

 one another completely and assume the aspect of dumb-bell shaped bodies. 

 Their number, however, has no significance, owing to their imperfect 

 form and individuality. The chromatin may remain for a considerable 

 period in one, or more than one, loosely formed mass. If it is more finely 

 divided, the fragments may be distributed with tolerable uniformity 

 throughout the cytoplasm (plate 5, figs. 30, 31a), or roughly aggregated 

 into two groups, one at each end of the cell. Not infrequently the chro- 

 matin bodies exhibit thread-like forms, especially in connection with what 

 appears otherwise to be anon-mitotic division of the polar cell (figs. 3 2, 3 3). 

 In no case, however, has it been observed that the chromatin is drawn 

 to the equator of a well-formed spindle and divided. Often the chromatin 

 fragments, especially the enlarged ends of the thread-like forms, show 

 vacuolation (figs. 30, 32, 33). Besides the deeply staining chromosomal 

 bodies, there are other less deeply staining bodies (figs. 2gb, 32, 33,34,35), 

 which apparently are modified chromatin; these occur either alone — 

 especially is this the case in small polar cells (figs. 34, 35) — or associated 

 with vacuolating parts (figs. 32, 33). These conditions all seem to point 

 to a degeneration of the chromatin. A nucleus is never formed, unless 



