158 The Life Story of the Fish 



member, and its duty is to go out from its place of origin in 

 the male, and to seek and enter the tgg. 



There are two basic methods for bringing about the union 

 of the two members. In the first and simplest the egg is set 

 free from the mother before it is fertilized. This occurs with 

 the oyster. Millions of eggs and sperms are released into 

 the water, and fertilization is entirely a matter of chance 

 encounter. This method can be used only by animals which 

 live in the water, for if the reproductive cells were released 

 in this casual way on land the male member, being capable of 

 movement only in a liquid medium, would be unable to seek 

 the female, and both would soon die. 



The second method, in use among both water- and land- 

 dwelling animals, overcomes this difficulty. The sperm is 

 introduced by the male into the body of the female, where 

 it encounters and fertilizes the tgg. This is a more efficient 

 method than the one first described, because the reproductive 

 cells of the two sexes are brought together in a restricted 

 space where their chance of encountering each other is in- 

 creased. It is found in such low forms as the flatworm, of 

 which the tapeworm occasionally parasitic in human stomachs 

 is an example, and in such high forms as the birds and human 

 beings. There is, however, this great difference: in the tape- 

 worm and the bird, the tgg is released after fertilization to 

 develop outside j in the human it remains within the body of 

 the female parent during the embryonic stages of develop- 

 ment, and the offspring does not emerge into the world until 

 it is a more or less finished product. In the bird, the tgg 

 contains abundant nutritive elements in the form of a yolk, 

 on which the embryo feeds during development 5 in the hu- 

 man and all other mammals the tgg contains comparatively 

 little yolk, and the embryo depends on the blood of the 

 mother for nourishment. It is not true, however, as many 

 people think, that the blood of the mother flows through 



