4 THE MATURATION OF THE EGG OF THE MOUSE. 



Although, he says, only 25 per cent of all the eggs form two polar 

 cells, all form two spindles, both of which divide; however, in those 

 eggs which have only one polar cell, it is the second polar cell which is 

 suppressed. This failure of the second polar cell to be formed is brought 

 about by a rapid division of the spindle. As a result the chromosomes 

 which would have been in the polar cell are retained in the egg cyto- 

 plasm, where they degenerate. The rapid division is, in turn, a conse- 

 quence of late semination. 



Gerlach finds that the two polar cells are separated by a varying 

 distance. This he explains as the result of the migration of the second 

 spindle from the point at which the first polar cell was formed. Semina- 

 tion interrupts the migration and causes the spindle to divide in the 

 position it may have reached when it was stopped, whatever that position 

 may be. He believes that in both divisions the chromosomes are divided 

 crosswise, but he thinks that, for theoretical reasons, one of the divisions 

 should be considered longitudinal (i.e., an equation division). The chro- 

 mosomes of the first spindle are tetrads, those of the second, dyads. 

 In one case he found what he considered a centrosome. The first polar 

 cell is larger than the second. 



Lams et Doorme (1907) deal chiefly with the cytoplasm. They, 

 however, describe both spindles. The second spindle is slightly smaller 

 than the first, but it can be identified only by the presence of the first 

 polar cell. They believe that both spindles divide and that two polar 

 cells are cut off in all cases. The ab strict ion of the first polar cell and the 

 formation of the second spindle from the chromosomes left in the egg 

 take place in the ovary. Ovulation occurs, then, during the stage of the 

 second spindle. The second polar cell is formed in the oviduct after 

 semination. They maintain that the second polar cell is larger than the 

 first, also that the first degenerates. Each spindle has twelve chromo- 

 somes; centrosomes may exist, though they are not regularly present. 



Kirkham (1907) believes that in all eggs two polar cells are formed, 

 the first always being produced while the ovum is in the ovary. In his 

 opinion the first and second spindles differ in the nature of their chromo- 

 somes, those of the first being tetrads, the second, dyads. The number 

 of chromosomes is twelve. Centrosomes occur at the poles of both spindles. 

 The first polar cell is larger than the second and different in chromatin 

 content. He assumes that in most eggs the first polar cell is forced 

 through the zona pellucida and is lost. 



Melissinos (1907), in his paper on the development of the mouse 

 makes, in passing, a few remarks on maturation. He thinks that 25 per 

 cent of the eggs form two polar cells, and he places the number of chromo- 

 somes at eight. But his figures are so diagrammatic and indicate such 

 poor fixation of his material that not much weight can be given to them. 



Since 1895 Sobotta (1907) has considerably changed his former views. 

 He now maintains that one-fifth (instead of one-tenth) of the eggs form 



