38 CHORDATE ANATOMY 



been secondarily acquired. But even within a small group of vertebrates 

 the yolk content of eggs may be highly variable, being apparently easil}- 

 susceptible to evolutionary change. In point of size and yolk content 

 the vertebrate egg has evidently had many ups and downs. 



In spite of the diversity of vertebrate methods of reproduction, an 

 evolutionary trend is clearly to be seen. There is a certain extravagance 

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

 only a small percentage of survival. 



The evolutionary tendency has been, by introduction of efBcient 

 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. Unquestionably the high 

 degree of efficiency which has been attained by the sauropsidan method 

 of reproduction and also by placental reproduction in mammals is some- 

 how correlated with the necessity of adaptation to the circumstances of 

 living on land and in air. The primitive fish methods would obviously 

 be impracticable. An aquatic larval stage in the development of a horse 

 or an elephant can hardly be imagined although, developing as it does 

 in the fluid-filled amnion, the terrestrial descendant of ancient aquatic 

 ancestors does spend its early life in a fluid medium. 



With increase in chance of survival 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 

 eggs than 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. 



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 inter-action 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 cor- 

 responding parts of the two cells because the limiting membranes of the 

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



