246 Basic Structure of Vertebrates 



about the primitive method — millions of eggs, perhaps, in a season, but 

 only a small percentage of survival. With increase in chance of sur- 

 vival there is reduction in number of eggs produced. This result has 

 the appearance of achieving economy, but there is perhaps room for 

 question as to just how and where the economy comes in. Does it cost 

 a cod any more to produce seven million minute eggs that it costs a 

 viviparous dogfish to bear four or five large "pups"? By either method 

 of reproduction the numerical status of the species may be maintained, 

 and so, as remarked above, the net results of the two methods are 

 equally good. 



The evolutionary tendency has been, by introduction of efficient 

 protective, nutritive, and respiratory arrangements, together with 

 parental care, toward the guarantee of the survival of every potential 

 adult. This tendency bifurcates and culminates in two very differently 

 specialized methods, one in birds, the other in mammals. Unquestion- 

 ably the essential peculiarities of avian and mammalian reproduction 

 were primarily correlated with the necessity of adaptation to the cir- 

 cumstances of living on land and in air. The primitive piscine methods 

 would obviously be impracticable. An aquatic larval stage in the devel- 

 opment of a horse or an elephant can hardly be imagined although, 

 developing as it does in the fluid-filled amnion, the terrestrial descend- 

 ant of ancient aquatic ancestors does spend its early life in a fluid 

 medium. But, as we observe the high degree of specialization and the 

 efficiency of these reproductive methods, we are inclined to feel that 

 they are somehow correlated also with a superior importance of the 

 avian and mammalian individual as contrasted with an individual 

 fish or amphibian. The bird and mammal are "higher" vertebrates. 



Development 



cleavage and blastula 



Development involves great protoplasmic activity. There must 

 be a building up of new protoplasm, rapid dividing of cells, movement, 

 and change of form. All of this calls for rapid metabolism. Metabolism 

 requires interaction of nuclear material and cytoplasm and exchange 

 of materials between the protoplasm and the external medium. The 

 area of the nuclear membrane and area of the external surface of the cell 

 therefore impose a limit on metabolic rate. Two cells are capable of 

 more rapid metabolism than one cell whose nuclear and cytoplasmic 

 volumes are respectively equal to the combined volumes of the corre- 

 sponding parts of the two cells, since the limiting membranes of the 

 two cells have greater total area than those of the single cell. 



The smallest egg-cells are large compared to most tissue-cells of the 



