204 



REDUCTION OF THE CHROMOSOMES 



the eighty-four dyads give rise to a reticular cleavage-nucleus. From 

 this arise eighty-four thread-like chromosomes, and the same munber 

 appears in later cleavage-stages. 



{b) It is the second and rarer mode that realizes Boveri's concep- 

 tion (Fig. loi). Both polar bodies are formed, the first removing 

 eighty-four dyads and leaving the same number in the egg. In the 



~^"m 





D 



/•: 



Fig. loi. — Second type of maturation in the parthenogenetic egg of Artetnia. [Brauer.] 

 A. Formation of second polar body. B. Return of the second polar nucleus {p.b:-) into the 

 egg ; devvlopment of the egg-amphiaster. C. Union of the egg-nucleus ( 9 ) with the second 

 polar nucleus {p.b?).^ D. Cleavage-nucleus and amphiaster. E. First cleavage-figure with 

 equatorial plate containing i68 chromosomes in two groups of 84 each. 



formation of the second, the eighty-four dyads are halved to form 

 two daughter-groups, each containing eighty-four single chromosomes. 

 BotJi these groups remain in the egg, and each gives rise to a sttigle 

 reticular nucleus, as described by Boveri in Ascaris. These tiva nuclei 

 place the VIS elves side by side in the cleavage figure, and give rise each 

 to eighty-four chromosomes, precisely like tivo germ-nuclei in ordinary 

 fertilization. The one hundred and sixty-eight chromosomes split 



