SEX DETERMINATION AND SEX-LINKED HEREDITY 405 



conditions disturb the normal course of events. For purposes of 

 practical genetics we may then define a female as an individual that 

 produces ova and a male as one that produces spermatozoa. 



Secondary sexual characters. — Usually males and females differ 

 from each other in many other characters besides the production of 

 eggs or sperm. Often one sex is larger, stronger, more elaborately 

 ornamented and colored than the other and possesses characteristic 

 accessory sex organs whose function it is to facilitate the bringing 

 together of the eggs and the sperm. All of the differences between the 

 sexes other than the primary difference of egg or sperm production are 

 called secondary sexual characters. Usually very young animals show 

 only slight differences in secondary sexual characters and the differ- 

 ences increase markedly at sexual maturity. We speak of the gradual 

 divergent development of the two sex types as sex differentiation. 

 The question arises as to whether or not the chromosomal differences 

 are the causes of the differentiation of secondary sexual characters. 

 These secondary sexual characters are all somatic, and, since the soma 

 is the product of cell division of the zygote, the soma cells must have 

 either the male or the female chromosomal character. That the 

 chromosomal mechanism in the somatic cells is not sufficient of itself 

 to bring about, unaided, the differentiation of secondary sexual charac- 

 ters can be shown readily in at least many animals. 



In the mammals, for example, it is known that the early removal 

 of the testes or ovaries results in a retention of the juvenile or undif- 

 ferentiated condition of secondary sexual characters. Evidently some 

 influence is exerted by the tissues of the gonad that is necessary for the 

 full differentiation of sex characters. The current theory is that 

 certain glandular cells that form part of the body of ovary or testes 

 excrete materials into the blood that stimulate various tissues in 

 different ways and produce dimorphic results. The specific sub- 

 stances produced by these glands are called "hormones," for want 

 of a better name. To test the efficiency of these hormones the crucial 

 experiment of taking out the gonads of a young rat or guinea pig and 

 implanting the gonad of an individual of the opposite sex has been 

 many times performed. For example, Steinach castrated young male 

 rats and then successfully grafted into them ovaries from young 

 female rats. The result was that these young rats which started to 

 be males became much altered in a female direction, the mammary 

 glands becoming greatly enlarged, their instincts more feminine than 

 masculine, and in a number of other particulars they showed more 

 or less pronounced evidences of feminization. Conversely, spayed 



