66 THE MATURATION OF THE EGG OF THE MOUSE. 



late stages (pronuclei and cleavage stages) , it presented few of the degen- 

 erate polar cells (see p. 41), those that persisted being of the larger size. 

 Again, eggs fixed in osmic-acid mixtures (which he used chiefly) have the 

 zona dark, which makes it difficult and often impossible to interpret or 

 even to see such small objects. Upon consideration, it is not surpris- 

 ing that the first polar cell should degenerate, for usually both polar cells 

 do so in time, forming no part of the embryo. It is quite possible that 

 the substance of the polar cell is absorbed by the egg. 



The decrease in size of the degenerating polar cell explains the dis- 

 agreement of authors concerning the relative size of the first and second 

 polar cells. Sobotta (1907, p. 536) maintains that sometimes one, some- 

 times the other, is larger. Gerlach (1906, p. 13) says the first is larger; 

 Lams et Doorme (1907, p. 287) that the second is. It seems fairly cer- 

 tain that Lams et Doorme must have seen old first polar cells and young 

 second ones, for they have few of the earlier stages, even though they 

 show the first polar cell decreasing in size. 



Gerlach (1906, p. 25) thought that in one first polar cell the chromo- 

 somes were dyads. Sobotta (1907, p. 537) says that both polar cells may 

 have either scattered chromatin or a nucleus, which is formed later than 

 the egg nucleus. In our opinion this statement must mean that he con- 

 fused the polar cells, for, of the 507 eggs with the second spindle that 

 we have studied, none have a first polar cell with a nucleus; whereas the 

 second, in seminated eggs, always forms a nucleus without its chromo- 

 somes becoming scattered and distinct. Kirkham (19076, fig. 14), also, 

 has probably mistaken the first polar cell for the second in the figure in 

 which he shows the monads much separated. 



The difference in chromatin contents of the two polar cells accords 

 with the well-known fact that the first polar cell corresponds to the first 

 oocyte, while the second is a homologue of the second oocyte; for, on the 

 one hand, the chromatin of the first polar cell does not form a resting 

 nucleus, but may divide (as it occasionally does), and, on the other hand, 

 the chromosomes contained in the second polar cell immediately become 

 metamorphosed into a nucleus corresponding to the egg nucleus. The 

 first, being a cell which degenerates, divides not regularly and normally, 

 but with what seems to be imperfect mitosis or even amitosis. 



5. REDUCTION. 



It is fair to assume from the preceding account that the longitudinal 

 division in the tetrads corresponds to the longitudinal split in the spireme 

 of a synapsis stage, and that the transverse division marks the place of 

 union, end to end, of two somatic chromosomes. Since the tetrad gives 

 rise to two dyads by parting along the transverse plane of division, and 

 since the dyads form their daughter chromosomes by means of the longi- 

 tudinal division, the maturation of the mouse egg belongs to the class of 

 prereduction divisions. 



