306 



ANIMAL BIOLOGY 



So it seems clear that sex is typically determined in many 

 animals, including Man, at the time of fertilization by the same 

 fundamental mechanism that controls inheritance in general. 

 Moreover, multiple genes are involved. The decision is given 

 by certain genes in the X chromosomes, acting in connection with 

 genes in other chromosomes. The genes in the X chromosomes 



X A 

 ABC 



Fig. 192. — Influence of the balance between X chromosomes and the other 

 chromosomes on sex in Drosophila. A, 'super female'; B, 'intersex'; C, 'super 

 male.' See Figs. 191, 197. 



turn the balance under the usual conditions of development so 

 that a series of processes is initiated, involving the action and 

 interaction of hormones, nutritional factors, and various envi- 

 ronmental conditions, that lead to the sex differentiation of the 

 adult. 



But many unusual cases, some normal and others abnormal, 

 occur particularly among the lower animals. Thus it has been 



found that intersexes, indi- 



cv iicc*|m 



If It •• ct re a cc 



• t f l CC Ct CC II |i 



B 



viduals exhibiting varying de- 

 grees of male and female 



characters, may result from 

 abnormal sets of chromosomes 

 in which the balance between 

 the X and the other chromo- 

 somes is upset, as is well il- 



A 



Fig. 193. — A, Human spermatogo- 

 nium with 48 chromosomes; B, chromo- 

 somes arranged to show the 24 pairs of lustrated by studies Oil Dro- 

 synaptic mates. The X-Y pair is at sophila . Qr hormones mav 

 lower right, (From Painter.) ^ 



have a modifying influence. 

 Thus a male twin in cattle may render abnormal the develop- 

 ment of its female twin in the uterus, so that a sterile ' free-martin ' 

 results. Or still again, sex reversal may occur through environ- 

 mental factors acting on the early embryo, as in the case of the 

 Frog. And finally, the sex of the adult may change, sometimes 



