244 Basic Structure of Vertebrates 



"milk." One egg or millions of them may be produced at a time. They 

 may or may not have shells. Parental care of eggs or young ranges from 

 nothing to the human maximum. Vertebrates may be oviparous or 

 viviparous. A primary oviparity may be succeeded by a secondary 

 substitute for viviparity, as when eggs develop within a fish's mouth, 

 an amphibian vocal sac, or integumentary pouches of various sorts. 

 Differentiation of organs may precede growth or it may be delayed 

 until the embryo is relatively large. The newly hatched larva of so 

 large a fish as the Atlantic salmon is about 0.65 inch long; a newborn 

 whalebone whale is about 20 feet long. The embryo may develop 

 directly to the adult form or there may be a larval period terminated by 

 a metamorphosis. The embryo may or may not produce a complex set 

 of temporarily functional membranes — amnion, chorion, allantois. 



The important point to be appreciated is that the association to- 

 gether of any two or more of these various alternatives in a single 

 animal is not haphazard. If one circumstance is, in itself, inadequate 

 for the success of reproduction, it is supplemented by something else. 

 If a large fish were to produce one single microscopic egg annually 

 and deposit it anywhere in the Pacific Ocean, the species would soon 

 become extinct. On the other hand, there is no unnecessary duplication 

 of highly specialized arrangements. A placental mammal does not 

 produce a large yolky egg. The entire complex of reproductive con- 

 ditions occurring in any one animal comprises a consistent grouping of 

 such alternatives as make it, as a whole, adequate. Despite the great 

 differences in methods of reproduction, the net results are equally 

 good, or nearly so, and generation after generation the life of the world 

 goes on with at most only very slow change in the general biologic 

 balance and scheme of things. 



Fishes and amphibians show this reproductive diversity most 

 markedly. Assuming a genetic series from fish to bird and mammal, the 

 evolution of reproduction has not been a direct progress along one 

 straight and narrow path. Instead, the animals within each class, 

 especially the lower, have tried (so to speak) a variety of methods. 

 From the many reproductive "experiments" of the lower vertebrates 

 finally emerge two distinct types to which the higher vertebrates ad- 

 here fairly closely. Reptiles and birds exhibit one of these types, 

 mammals the other. Yet certain distinctive features of these finally 

 emergent types of reproduction are anticipated by some lower verte- 

 brates. The enormous eggs of oviparous sharks and skates, encased in 

 thick shells, resemble eggs of reptiles. Some viviparous sharks produce 

 vascular uterine structures (see p. 239) suggestive of the mammalian 

 placenta. Certain viviparous lizards (genus Seps) develop what is 

 practically a placenta. But there can hardly be any direct genetic 



