THE HORMONES IN HUMAN REPRODUCTION 



record of evolution has some torn-out pages at this point) 

 that an animal first became sufficiently complex to set aside 

 a germ cell, specialized for reproduction. Had a scholastic 

 philosopher been present on that prehistoric occasion the 

 only question would have been which he noticed first — prob- 

 ably not the egg, because it was smaller than the rest of the 

 animal. 



The sperm cell. Hydras do not, however, always form eggs ; 

 half the time they develop not an ovary, but a testis, in which 

 a few cells of the animal give rise by repeated division to a 

 large number of very small sperm cells (diagram. Fig. 5, 

 right; and photograph, Plate II, C). These cells can swim 

 independently, when they are discharged into the water, by 

 means of a motile tail with which each is provided. The sole 

 function of such a cell is to swim until it meets an egg cell 

 released from another Hydra and to enter it. When the egg 

 is thus "fertilized" by union with a sperm cell, and then only, 

 it begins to divide and ultimately to become a new Hydra. 

 Herein we have the elements of sex, for this new polyp has two 

 parents, which were not exactly alike in spite of their general 

 similarity, because one of them furnished the egg and was 

 temporarily at least a female ; the other, which furnished the 

 sperm cell, was temporarily a male. 



THE MEANING OF SEX 



No characteristic of man and the other animals is so 

 fundamental, so completely taken for granted, as the exist- 

 ence of two sexes. It is the first fact the Bible mentions about 

 the human race: ". . . male and female created He them." 

 In every nature myth the animals enter two by two. In 

 primitive song and story every Jack that cracks his crown 

 has a Jill that tumbles after. Man that is born of woman finds 

 it impossible to think of a race with only one sex, or to 

 imagine other sexes than two. Nor does the biologist contra- 

 dict this axiom; everywhere in nature he also sees two sexes. 



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