164 The Life Story of the Fish 



gradations to pure examples of method Number 3, where the 

 tgg contains practically no yolk, and all nourishment for the 

 developing young is received from the mother. The means 

 by which this is accomplished are diverse. In some the young 

 fish, free in the ovarian cavity, merely absorb maternal 

 fluids. In others, processes are formed which bring the mem- 

 branes of mother and child in close contact j or flaps grow 

 from the mother's tissues into the gill openings of the baby, 

 or from the anal orifice of the baby to the ovarian walls. And 

 in one instance, the embryo actually feeds on the mother, 

 for in its digestive tract have been found traces of her 

 broken-down follicular tissue, and, in addition, of dead 

 sperms which had entered the ovarian passages without ful- 

 filling their mission, as well as the remains of other embryos 

 which had presumably failed to survive after development 

 had started. Here is cannibalism at an early age indeed! 



Method Number 3, then — ^internal fertilization and in- 

 ternal development — has in fish evolved from Number 2. 

 It has evolved separately and in different ways in different 

 groups. And it is in the primitive sharks that it has most 

 closely approached the mammal. Internal fertilization is 

 common among sharks, the male introducing the sperm into 

 the female with modifications of his ventral fins called 

 "claspers." In many species the eggs are then extruded into 

 the water, a pure example of method Number 2; but in 

 some there is true internal development, and in at least one 

 species the blood stream of the embryo and that of the 

 mother have been brought into a relationship every bit as 

 close as that found in the placenta of the mammal. If this 

 primitive fish had some means of furnishing its infant with 

 nourishment from its own body after birth it would be 

 reproductively on a par with us. 



However, this is a far-flung exception. In the great ma- 

 jority of fish, fertilization takes place in the water and the 



