DIFFERENTIATION, HEREDITY, SEX 307 



Differences of these kinds are now known in many scores of 

 species of many groups, from the lower worms to man. It is 

 clearly impossible to include here any extended history or 

 survey of this fascinating subject and we can do little more 

 than describe a typical instance or two, and then mention 

 some comparisons which may throw some light, from this point 

 of view, upon the general interpretation of the chromosome 

 problem. 



Since the larger number of the known instances of this rela- 

 tion are found among the Arthropoda, particularly the Insecta, 

 we may select our first illustration from this group. The num- 

 ber of chromosomes in the somatic cells of the common squash 

 bug, Anasa tristis (Henking, Paulmier), is twenty-two in the 

 female, and twenty-one in the male. How does this difference 

 come about? 



For an answer to this question we must observe the behavior 

 of the chromosomes during the process of maturation of the 

 germ cells. In the process of oogenesis, preparatory to the first 

 oocyte division, synapsis occurs normally, and eleven bivalent 

 chromosomes are formed. The succeeding steps in oogenesis 

 are not unusual and the result is the formation, in each ovum, 

 of a group of eleven univalent chromosomes representing every 

 pair of the original somatic or oogonial group. 



The events of spermatogenesis do not run quite parallel, 

 however. In the somatic and spermatogonial cells, twenty-one 

 chromosomes are present, i.e., ten pairs plus one, and in the 

 division of these cells every chromosome divides in the usual 

 way (Fig. 142). In synapsis the paired elements fuse, forming 

 ten bivalent chromosomes, but the odd chromosome, or X- 

 chromosome remains free, and usually quite apart from the 

 other chromosomes. This X-element, or idiochromosome, may 

 be distinct, even throughout the growth period of the spermato- 

 gonia, and during the two spermatocyte divisions it can be 

 identified in many species as a nucleolus-like body, indeed 

 formerly it was described as a chromatin nucleolus. The behav- 

 ior of this body during the maturation divisions is entirely un- 

 _usual. During the first spermatocyte division the idiochro- 



