GENETICS 253 



of animals, especially among the insects. This method of sex-determina- 

 tion will be described. 



In most insects the male and female are characterized by different 

 numbers of chromosomes. In a certain species of bug the female has 22 

 chromosomes in every cell of her body, the male only 21.^ Now, as was 

 pointed out in the chapter on embryology, in the maturation of the germ 

 cells, the number of chromosomes is reduced to half. In the female of 

 the species here referred to, each mature egg thus receives 11 chromosomes. 

 In the male, with its odd number, an equal division of the chromosomes 

 into two groups is out of the question. Twenty of his chromosomes unite 

 in pairs in the early stages of maturation, and produce ten tetrads. The 

 twenty-first chromosome never pairs with another chromosome, nor 

 produces a tetrad. 'In one of the maturation divisions it passes undivided 





in; ,11;, . *♦%. 



>ii 



A 



a 





Fig. 201. — The maturation divisions of the male germ cells of the bug Anasa. a, 

 polar view of metaphase of first division; all the chromatic bodies are double except one, and 

 therefore represent 21 chromosomes, the somatic number; 6, second maturation division 

 in side view, not all of the chromosomes shown ; the single chromosome of a is going un- 

 divided to the lower pole; c and d, polar view of the two anaphase groups of the second 

 division; 11 chromosomes go into one spermatid (female-producing), 10 into the other 

 (male-producing). {After Wilson in Journal of Experimental Zoology.) 



to one daughter cell. Figure 201 represents the reduction division. As a 

 result, there are produced two kinds of spermatozoa. HaK of them have 

 10 chromosomes, half of them 11. When an egg (with 11 chromosomes) 

 is fertihzed by a spermatozoon having 11 chromosomes, the resulting 

 individual has 22 chromosomes, and becomes a female. When an egg 

 is fertilized by a spermatozoon having 10 chromosomes, the individual 

 developing from the fertilized egg has 21 chromosomes, and is a male. 

 In insects whose sex is determined in this way, at the time of fertilization, 

 all efforts to alter the sex subsequently have failed. 



Some plants have an equally infallible method of sex-determination. 

 Some of the mosses have the sexes regularly separated. That is, one 

 plant will produce only eggs (female), another plant only sperms (male). 

 The fertilized egg develops into a sporophyte in which numerous spores 



