74 



GENERAL EMBRYOLOGY 



malities occur in the distribution of the chromosomes, and it is always 

 the case that the number of chromosomes forming out of a nucleus is the 

 same as the number passing into it, no matter how that deviates from the 

 normal (Boveri) (Fig. 38). There seems to be no regulation within the 

 nucleus in this respect, such as would result were it a unified structure 

 tending always to maintain its own normal. In many instances of 



D 



FIG. 38. Indications of the individuality of the chromosomes in the fertiliza- 

 tion of Ascaris. From Wilson, "Cell," after Boveri. A. The two chromosomes 

 of the egg-nucleus, accidentally separated, have given rise each to a reticular nu- 

 cleus (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; p.b.* the two chromosomes arising from 

 it, 9 the egg-chromosomes, cT the sperm-chromosomes. D. Resulting equator- 

 ial plate with six chromosomes. 



alternation of generations, the agametically produced generation is 

 formed from a cell with - chromosomes; in all the cells of such an 



organism, the nuclei show the same reduced number, even in so 

 complex an organism as the fern prothallus. In some forms, the 



