76 GENETICS 



Summary of Possible JVays of Determining Sex 



I. Selective action exercised on the germ cells containing 

 particular chromosomes. — In the organisms of Group I, 

 there are two kinds of sperms; those containing an X- 

 chromosome (which produce females), and those contain- 

 ing no X, which produce males. Any agent that would de- 

 termine whether certain eggs are fertilized by an X sperm 

 or by one without an X would determine the sex. There are 

 a number of possibilities of this sort. 



(a) It Is known that the sperms containing an X-chromo- 

 some are in some animals larger than those without X. It 

 may well be therefore that one kind of sperm is more vig- 

 orous and active than the other. This kind would then 

 fertilize a greater proportion of eggs than the other, so that 

 a majority of the offspring would be of one sex rather than 

 the other. 



There are indications that something of this sort is true 

 in certain organisms. Males and females are not produced 

 in equal numbers in all organisms. The average number of 

 males to every 100 females are for certain species as fol- 

 lows: 



Man 105, Cattle 107, Pigeon 115, Cottus (fish) 188, 

 Lophius (fish) 385, Squid 17, Octopus 33. (Data from 

 Goldschmidt, 1923.) 



Such disproportion between males and females might 

 also be produced in some of the other ways hereinafter 

 mentioned. 



(b) It is known that in some organisms all the sperms of 

 one type die. In a species of Phylloxera, according to Mor- 

 gan,^ at a certain period in the life history all the sperms 

 that do not contain X die. As a result, all of the offspring 

 p-oduced In the next generation are females. 



(c) If one of the two types of sperm is more resistant 



