CHAPTER XXIII 



CHROMOSOMES AND SEX 



During the closing decades of the nineteenth century many researches 

 were carried out in the hope of identifying the controlUng agency in 

 sex determination with one or more of the environmental factors. The 

 effects of light, temperature, moisture, and nutrition were examined, 

 and although a number of workers believed their methods to be in a 

 certain measure successful, the results were on the whole inconclusive. 

 The period during which sex was thus looked upon as a character more or 

 less under the control of the environmental factors was succeeded by one 

 in which it came to be regarded as something regulated mainly by some 

 internal mechanism or condition, and as relatively unalterable by external 

 agencies. This conclusion, definitely reached by Cuenot (1899) for 

 animals and by Strasburger (1900c) for plants, received the support of a 

 number of experimental researches on animals and dioecious plants. It 

 was further strengthened by the discovery that in many organisms males 

 and females differ visibly in their chromosome complements. 



The modern interpretation of sex determination combines the elements 

 of truth in both of these theories. The course of development in an 

 organism is dependent on a series of interactions between a protoplasmic 

 system of a certain kind and a particular group of environmental con- 

 ditions. It is found that the differentiation of sex, like that of any other 

 character, involves the action of both internal and external factors. In 

 some cases the tendency to develop one sex or the other is so strongly 

 set by the internal factors that it can be modified only with difficulty, 

 if at all, by altering the external factors; whereas in other cases the 

 internal factors are so balanced that the course of differentiation can be 

 thrown toward either sex with comparative ease by altering the environ- 

 ment in which the individual develops. 



Sex-chromosomes in Animals. — In a great many animals it has been 

 found that the approximate numerical equality of males and females is 

 related to the fact that the gametes of one sex (more commonly the male) 

 are of two kinds, the sex of the offspring depending upon which kind 

 happens to function in syngamy. The difference in the two kinds of 

 gamete is often due to the meiotic disjunction of the unlike members of a 

 distinguishable pair of chromosomes which evidently play a special role 

 in sex determination. Such chromosomes are known as sex-chromosomes; 



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