Sex Determination (/) 



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duced in separate gonads located in different 

 segments of the body. Similarly, in certain 

 mosses, egg and sperm-like gametes are pro- 

 duced in separate sex organs (located on the 

 same haploid gametophyte individual). 



In all these cases, the organism which pro- 

 duces the two types of gametes started its 

 existence by containing but a single geno- 

 type. It might be supposed, at first, that the 

 haploid genotype carried by eggs and by 

 sperm is different, and is the cause of the 

 difference in their phenotype and behavior. 

 But, in the case of the gametophyte of mosses, 

 the individual is haploid and so are both types 

 of gametes it forms. Accordingly, we cannot 

 expect differences in gene content to be the 

 basis in such organisms either for the forma- 

 tion of gametes or for the different types of 

 gametes produced. 



Gamete formation in hermaphroditic and 

 monoecious organisms, therefore, is depend- 

 ent primarily upon environmental differences. 

 These environmental differences must exist 

 even between cells which lie close together, 

 as is the case in Helix, and must often result 

 from differences in the internal environment 

 of an organism. We shall not pursue much 

 further the matter of identifying the nature 

 of the differences in the internal (or external) 

 environment which result either in gamete 

 formation or in the specific types of gametes 

 formed. It will suffice, here, to point out that 

 these sexual events are but specific instances 

 of the general process of cellular differentia- 

 tion that occurs in multicellular organisms. 

 It is reasonable to suppose that the same 

 kinds of environmental factors which can 

 direct one group of cells to form muscle cells, 

 and an adjacent group to form bone cells, 

 could operate to direct the differentiation of 

 still other cells into gonadal tissue in which 

 adjacent cells might further differentiate as 

 sperm and egg. You should note, however, 

 that there is another problem of differentia- 

 tion involved in sex, which is separate, at 

 least in some organisms like the mosses, from 



the production of gametes — the cells uniting 

 in fertilization. This problem, which we shall 

 not discuss further, concerns the genetic and 

 environmental factors responsible for the on- 

 set of meiosis, which is, of course, the feature 

 most fundamental to the success of the sexual 

 process as it occurs at present. 



The examples already mentioned dealt with 

 the dependency of gamete differentiation upon 

 the different positions which cells have within 

 a single organism, as a consequence of which 

 these cells are subject to differences in internal 

 and external environments. In the marine an- 

 nelid, Ophryotrocha, the two sexes are in sep- 

 arate individuals and the sex type formed is 

 determined by the size of the organism. When 

 the animal is small, because of youth or be- 

 cause it was obtained by means of amputat- 

 ing a larger organism, it manufactures sperm; 

 when larger, the same individual shifts to the 

 manufacture of eggs. In this case the en- 

 vironment of the gonad is changed by the 

 growth of the organism. Finally, we can 

 consider the determination of sex in the 

 marine worm, BoneUia. In this organism the 

 separate sexes are radically different in ap- 

 pearance and activity, females being walnut- 

 sized and having a long proboscis, males 

 being microscopic ciliated forms that live as 

 parasites in the body of the female. Ferti- 

 lized eggs develop into females when grown 

 in the absence of adult females; they grow 

 into males in the presence either of adult 

 females or simply of an extract of the pro- 

 boscis of females. In this case, then, differ- 

 entiation as a whole, including sexual differ- 

 entiation, is regulated by the presence or 

 absence, as part of the environment, of a 

 chemical messenger manufactured by females. 



Nothing has been stated regarding the 

 specific genetic basis for the determination or 

 differentiation of sex in the examples so far 

 discussed. Different sexes or gametes were 

 determined not by genetic differences between 

 cells, organs, or individuals, but by environ- 

 mental differences acting upon a uniform 



