2l6 



SOME PROBLEMS OF CELL-ORGANIZATION 



the egg-nucleus. At a later period this nucleus gives rise to 

 the same number of chromosomes as those that entered into its 

 formation ; i.e. either one or two. These are drawn into the 

 equatorial plate along with those derived from the germ-nuclei, and 

 mitosis proceeds as usual, the number of chromosomes being, how- 

 ever, abnormally increased from four to five or six (Fig. 103 C, D). 

 Again, the two chromosomes left in the egg after removal of the 



D 



Fig. 103. Evidence of the individuality of the chromosomes. Abnormalities in the fertiliza- 

 tion of Ascaris. [BOVERI.] 



A. The two chromosomes of the egg-nucleus, accidentally separated, have given rise each to a 

 reticular nucleus (9, 9) ; the sperm-nucleus below (c?). B. Later stage of the same, a single 

 chromosome in each egg-nucleus, two in the sperm-nucleus. C. An egg in which the second 

 polar body has been retained; /. b: 1 the two chromosomes arising from it, $ the egg-chromo- 

 somes, o" the sperm-chromosomes. D. Resulting equatorial plate with six chromosomes. 



second polar body may accidentally become separated. In this 

 case each chromosome gives rise to a reticular nucleus of half the 

 usual size, and from each of these a single chromosome is afterwards 

 formed (Fig. 103, A, B). Finally, it sometimes happens that the two 

 germ-nuclei completely fuse while in the reticular state, as is nor- 

 mally the case in sea-urchins and some other animals (p. 153). From 

 the cleavage-nucleus thus formed arise four chromosomes. 



