DEVELOPMENT. 47 



results in an equal division of the chromosomes. 

 What these unrevealed biological units are is a 

 matter of much controversy. 



Much work has been done in recent years on the 

 individuality of the chromosomes, establishing the 

 fact that they vary greatly in size, form, and con- 

 stituents in one and the same cell. Some are long 

 and some are short, thick and slender, greatly 

 curved and nearly straight, stain dark or take only 

 a slight stain; in short, we must now admit their 

 personnel and postulate special cell function for 

 each chromosome. Moreover, it is now practically 

 established that certain spermatozoa, particularly 

 in insects, have an odd chromosome, and that from 

 eggs fertilized by this particular class males de- 

 velop, whereas from fertilization by spermatozoa 

 having an even number of chromosomes females are 

 produced. In a species of insect, Lygceus bicrucis, 

 the males develop spermatozoa that have all the 

 same number of chromosomes, but in one set there 

 is present one large chromosome and in the other 

 set there is present one small chromosome. If a 

 spermatozoon with the small chromosome fertilizes 

 the egg, a male develops, and if one with the large 

 chromosome fertilizes it, then a female is formed. 

 Sex chromosomes have now been recognized in a 

 large number of males of different groups of ani- 

 mals, and we can, therefore, safely affirm that the 

 determination of sex rests, in most cases, with the 

 intrinsic quality of the fertilizing spermatozoon. 

 Morgan, therefore, concludes that "if these observa- 



