Reproduction and Growth 159 



the veins of her baby; what happens is that the blood ves- 

 sels of the mother are brought into such intimate contact 

 with the membranes surrounding the developing child that 

 the necessary nourishment passes from one to the other 

 through the very thin and permeable walls. 



Boiled down to their elements, there are three basic sys- 

 tems: (i) external fertilization and external development 5 

 (2) internal fertilization and external development j (3) 

 internal fertilization and internal development. 



Fish are able to make use of all three of these methods. 

 They are the only vertebrates, aside from the amphibians, 

 which use method Number i. They utilize method Number 

 3 in a form as fully developed as the mammals. And they 

 have made modifications in method Number 2 until it is at 

 times unrecognizable. 



The great majority of them favor Number i. Some of 

 the species have made refinements on the old system, but 

 others use it in all its original simplicity. That little cousin 

 of the tuna, the mackerel, is an example: males and females 

 come together in great schools at the spawning season and, 

 indiscriminately and without any apparent mating, release 

 their reproductive cells into the water. It is up to chance to 

 do the rest. Whether sperm meets egg, and whether fer- 

 tilized egg survives and develops into a new mackerel, is of 

 no more concern to the parent mackerel than if it were an 

 oyster. The term "poor fish" seems fully justified here, for 

 there appears to be no consciousness of sex, no gratification 

 from the sex act — nothing more than a relief from the pres- 

 sure of overabundant sex products. 



The trout do better. Here there is an unmistakable pair- 

 ing, a preparation of the bottom for the eggs by the female, 

 an accompanying defense by the male, and a collaborative 

 sex act, as we shall see in a later chapter. When the whole 

 process is finished the parents separate and go about their 



