REPRODUCTION 



537 



become parents at all, so that with their 

 death their indifference would also become 

 extinct. 



Evolutionary significance 



Viewing the vertebrates in a s;eneral 

 way, it is true that giving birth to active 

 young is more advanced than egg laying. 

 There are, however, a number of very inter- 

 esting primitive vertebrates, the sharks, for 

 example, that have progressed a long way 

 toward producing and caring for young in 

 much the same way mammals do. Some 

 sharks inhabit the deep sea and never come 

 into shallow waters to deposit their eggs in 

 safe places. In such species, the eggs are re- 

 tained in the uteri where they can be better 

 cared for. In one small shark (Mustehis) 

 tiny projections, like villi in the intestine, 

 protrude from the yolk sac and penetrate 

 the uterine wall from which the embryo 

 derives nutritious secretions (Fig. 21-5). 

 This condition certainly approaches the 

 placenta of mammals; indeed, these sharks 

 are called "placental" sharks. Another in- 

 teresting device for extra-egg nourishment 

 of the developing embryo is found among 

 the rays, close relatives of sharks. In some 

 of these animals, glandular teats grow out 

 from the inner uterine wall and by con- 

 tractions force their secretion into the 

 mouth or spiracle of the embryo, thus re- 

 sembling extrauterine feeding of mammals. 

 In spite of these rare cases, it is generally 

 true that evolution of the reproductive sys- 

 tem among vertebrates has been from the 

 egg-laying forms to tliose that retain and 

 nourish their young within the uterus of 

 the mother. 



We can make one further generalization, 

 constantly keeping in mind the occasional 

 exception which prevents one from draw- 

 ing final sweeping conclusions. Most primi- 

 tive animals make headway by sheer 

 weight of numbers; millions are produced 

 but only a small fraction of a per cent sur- 

 vive to maturity. As the animals become 

 more complex, fewer and fewer offspring 



are produced but the early mortality rate 

 drops also. With smaller numbers greater 

 parental care is given, so that the percent- 

 age of survival is much greater. The fish de- 

 posits its eggs in a scooped-out hollow on 

 the stream bottom and immediately leaves 

 them to the ravages of other fish. A large 

 mammal like a horse bears young only once 

 a year but gives the offspring great care, 

 even to the point of sacrificing her own life 

 (Fig. 21-2). In each case the chance for 

 race survival may be the same. Immediately 

 we can think of exceptions to this gener- 

 alization. The codfish, for example, lays 

 millions of eggs each season, whereas the 

 shark may have no more than a dozen in an 

 equal period of time, yet the shark is more 

 primitive than the cod. Frogs and toads 

 give their young no care whatever, but the 

 bluegill (fish) will protect its nest of eggs 

 viciously against all intruders. These are 

 good examples to remind us of the caution 

 one must exercise in stating a generaliza- 

 tion. 



THE HUMAN REPRODUCTIVE 

 SYSTEM 



Although there are minor differences in 

 the reproductive apparatus among various 

 mammals, they are all essentially alike, man 

 included. One rather striking variation has 

 undoubtedly had considerable influence on 

 man's habits and indeed his entire social 

 structure. That is the lack of a seasonal or 

 periodic breeding season. The sexes are 

 mutually attractive throughout the year. 

 This fact probably has had some bearing on 

 the establishment of permanent unions that 

 could provide for offspring which required 

 such long periods of time for growth to 

 maturity. This is the basis of the family 

 which, in turn, underlies our whole social 

 order as we know it in civilized nations to- 

 day. It should not be implied, however, that 

 man is by nature fitted perfectly into the 

 strait jacket of civilization he has fashioned 

 for himself. In fact, he is constantly in 



