58 COMPARATWE ANATOMY 



the yolk content of eggs may be highly variable, being apparently easily 

 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 ef&cient protective, nutritive and respiratory arrange- 

 ments, together with parental care, to assure the survival of every potential 

 adult. This tendency bifurcates and culminates in two very differ- 

 ently specialized methods, one in birds, the other in mammals. With 

 increase in chance of survival there is reduction in number of eggs pro- 

 duced. 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 main- 

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

 equally good. 



Unquestionably the high degree of efficiency which has been attained 

 by the sauropsidan method of reproduction and also by placental repro- 

 duction in mammals is somehow correlated with the necessity of adapta- 

 tion 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. But it is beyond question also that the specialized repro- 

 duction of the bird and the mammal emphasizes the importance of the 

 individual animal. The evolution of these highly specialized reproduc- 

 tive methods may very reasonably be regarded as somehow correlated 

 with the fact that the individual bird or mammal is more important. 



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 



