THE GENERAL SCHEME 



ground, but have male germ cells that cannot move of their 

 own accord, the winds or the insects transport the pollen. 



Most of those animals that are free to move mate by pro- 

 pinquity. They can at the very least deposit their eggs and 

 sperm cells at the same place. This is the case in many fishes, 

 in which the male and female place themselves close together 

 when they spawn, so that the sperm cells are deposited upon 

 the eggs. Frogs and toads provide an even better chance of 

 contact between the germ cells, for in the mating season the 

 male instinctively clasps the female with his fore limbs and 

 the two animals remain in close contact for days, until the 

 eggs are discharged, whereupon the sperm cells are deposited 

 directly upon the eggs. In the tailed amphibians (such as 

 newts and salamanders) sperm cells are not discharged ex- 

 ternally at all. Like all other vertebrates below the mammals 

 (i.e. fish, amphibians, reptiles, and birds) these tailed am- 

 phibians have a combined cloacal passage into which both 

 the intestinal and the genital canals open. The openings of 

 the cloacas of the male and female are placed so close together 

 in mating that the sperm cells pass directly from one to the 

 other. They then pass up into the oviduct and fertilize the 

 eggs there. Much the same process occurs in many birds, 

 for example the common fowl and their kin. 



An obvious advance is the development of an organ for 

 direct transmission of the sperm cells. The elasmobranch 

 fishes (sharks and rays) possess specially modified anal fins, 

 called claspers, which are grooved so that the seminal fluid 

 containing the sperm cells is guided along them from the 

 cloaca of the male to that of the female. This particular 

 method of solving the problem is, of course, not available to 

 land animals, since they have no fins. In snakes and lizards 

 there are saclike branches of the cloaca that can be turned 

 inside out and protruded into the cloaca of the female, carry- 

 ing with them the sperm cells. In other reptiles, namely turtles 

 and crocodiles, and in many birds the final solution was 



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