36 CHORD ATE ANATOMY 



In placental mammals, as compared to marsupials, the young are 

 born at a relatively advanced stage of development and growth. The 

 mammary organs, however, are in all cases an important post-natal 

 provision for bringing the young animal along to a degree of size and 

 strength favorable to ultimate success. They afford the great advantage, 

 too, that the young animal is not thrown upon the world abruptly, but 

 may acquire independence gradually. 



Evolutionary Significance 



Surveying the whole group of vertebrates, the great diversity in the 

 conditions and arrangements attending reproduction is most impressive. 

 It would be difficult to imagine any practicable reproductive expedient 

 or condition which is not exhibited by some animal. There are micro- 

 scopic eggs and there are ostrich eggs. The quantity of yolk may be 

 vast or it may be next to nothing. The primary food supply, yolk, may 

 in various ways be supplemented by secondary sources of nutriment — egg 

 albumen, maternal blood, mammary milk, pigeon "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 ovi- 

 parity 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 new-born whalebone whale is about twenty 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 together 

 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 any- 

 where 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 conditions occurring in any 

 one animal comprises a consistent grouping of alternatives such that, 

 as a whole, it is adequate. Des[)ite the great differences in methods of 

 reproduction, the net results are equally good, or nearly so, and generation 



