208 AN INTRODUCTION TO MODERN GENETICS 



to the end, when the facts will be available on which a discussion 

 should be based. Meanwhile the word sex will be used in three senses : 

 (i) for the differentiation of the gametes into different kinds; (2) for the 

 differentiation of the body of the haploid organism into kinds bearing 

 different sexual organs ; (3) for the differentiation of the diploid organ- 

 ism in the same way. Organisms, whether diploid or haploid, which 

 bear both types of sexual organs and produce both types of gametes, 

 will be called bisexual, or monoecious or hermaphrodite, and here the 

 concept of sex is applied to the individual organs rather than to the 

 body as a whole. 



The second and third types of sexuaUty have been defined in terms 

 of haploid and diploid organisms. This requires modification. The 

 alternation of generations in a plant is essentially an alternation between 

 two morphological phases and is not absolutely coupled with a definite 

 chromosome cycle. Thus gametophytes can be produced, in mosses, 

 for instance, which are diploid instead of haploid (p. 213). Similarly, 

 the sex of a male bee, which is haploid, is of the same order as the sex 

 of the diploid female, since both obviously belong to the same morpho- 

 logical phase in spite of their difference in chromosome number. 

 Mainx has therefore proposed the terms Gamophase and Zygophase to 

 replace haploid phase and diploid phase above, and this suggestion will 

 be adopted here. The first kind of sexuality mentioned above will be 

 called Gamete sexuality. 



A. GAMETE SEXUALITY 



In organisms with zygophase sexuality, the question of whether a 

 cell in a sexual organ develops into a male or female gamete or gameto- 

 phyte seems never to be decided by its own genetic constitution, but 

 always by influences coming from outside the cell. It is a particular 

 case of differentiation during development and is decided like other 

 differentiations. Thus in vertebrates it is the result of a process of 

 induction : any primordial germ cell can develop either into an egg or 

 a sperm, and the way it does develop is determined by whether it is 

 acted on by the cortex of the gonad, which is highly developed in 

 females and induces eggs, or by the medulla, which is developed in 

 males and induces sperm. ^ Humphreys has been able to demonstrate 

 this directly by grafting male gonad-forming mesoderm over the pre- 

 sumptive germ-cells of a female in the Axolotl; in spite of their female 

 * Rev, Witschi 1934. 



