192 



ON VARIABILITY AND ADAPTATION 



The nucleus of the fertilized ovum or new individual is formed 

 of corresponding amounts of nuclear matter (chromatin) from 

 both parents. 



5. Not only is this the case, but most significantly — I shall 

 take up a probable exception immediately — each supplies a 

 like number of chromatin loops or chromosomes, and as the 

 fertilized ovum undergoes development and proceeds to divide 

 and redivide, the like process of distribution is continued, so 

 that each separate body cell of the fully developed organism 

 contains equivalent parts of chromatin of paternal and maternal 

 origin. 



6. We can proceed yet further and recognize that in certain 

 species at least the chromosomes supplied by or derived from 

 either parent while pairing with like chromosomes from the 



other parent are not 

 all identical in appear- 

 ance and size, but vary 

 among themselves, the 

 variation being con- 

 stant ; that is to say, 

 the same types of 

 chromosomes are found 

 in successive genera- 

 tions of cells. This 

 peculiar variation, as 

 has been pointed out 

 more particularly by 

 American observers 

 (Montgomery and 



Fig. 13. — Spermatocytes from human testis (after 

 Moore and Arnold) to demonstrate the various 

 forms of chromosomes contributed to the future 

 spermatozoa, and the presence of the various • 

 types in the different cells. These distinct types Sutton), is frequent in 

 a, b, c, d are always found in the human sperm • . • ,i 11 



mother cell when undergoing maiotic division. mseCCS in tne Cells 



which ultimately give 

 rise to the germ cells. As Moore and Arnold of Liverpool have 

 just shown, a like constancy is to be made out in the types of 

 chromosomes seen in the spermatocytes of mammals, even of 

 man himself. The constancy of the particular varieties present 

 in individual species suggests that the chromosomes of different 

 orders possess different properties and determine different 

 characters, or sets of characters, in the cells to which they are 

 distributed, and in the individual formed from the aggregation 



