Reproduction and Growth 163 



a bond between the mates which often outlasts the period 

 during which the young need care, and holds the parents 

 together for another spawning. In some cases it is very diffi- 

 cult to persuade either member of a couple to accept another 

 mate, even during the intervals between successive broods, 

 and although absolute monogamy has never been proved 

 there have been instances which seemed to bear all the ear- 

 marks of that sacred institution. 



Method Number 2, internal fertilization with external 

 development of the eggs, is used among more animal groups 

 than either of the two others. It is common among the in- 

 vertebrates, and it is the only method employed by the rep- 

 tiles and the birds. It is not uncommon among fish, but only 

 rarely does it occur in its pure and easily recognizable form, 

 in which the fertilized egg is released from the body of the 

 mother and hatches outside. In some of the rockfishes of the 

 Pacific coast, this is the case, but in other species of this 

 group the egg is retained inside the female and hatches 

 there, and what emerges is a young fish. While this would 

 appear to be a form of method Number 3 (internal develop- 

 ment), it is not truly so as long as no nourishment is received 

 from the mother. In the rockfish there is no more connection 

 between the developing embryos and their mother than there 

 is between a hen and the eggs on which she is sittings the 

 female fish's body is merely the nest in which the eggs re- 

 main until they hatch. She sits on her eggs not by covering 

 them with her body but by keeping them inside her body. 

 Her eggs, just like the hen's, draw upon their own yolk, not 

 upon their mother's blood stream, for nutrition. Develop- 

 ment of the eggs is external to her tissues, even though it 

 takes place within her body cavity. 



In GambusMy the well-known mosquito fish, the process is 

 similar; in the guppy of home aquarium fame, only slightly 

 modified; and from there on we proceed through various 



