i 9 6 AGE, GROWTH, AND DEATH 



ample and adequate nourishment of the developing 

 germ. By means of the placenta the uterine period 

 has been lengthened, and the embryo is nourished at 

 the expense of the mother with little physiological 

 exertion on its own part. Moreover, in all mammals 

 lactation and maternal care serve to further prolong 

 the developmental period. By these means the pro- 

 tective processes have been wonderfully perfected and 

 the result is that in mammals there is a long period 

 during which the production of cells goes on ; the 

 cells at first all remain relatively simple, and by the 

 time they begin to change the number of cells is so 

 great that the possibilities of an almost infinite variety 

 of peculiarities in them are given. There are cells 

 enough to allow this variety to be worked out. This 

 type of development we call the embryonic. We 

 know, therefore, that nature has recognised a restric- 

 tion which she herself has put upon development, the 

 restriction which obliges development, if it is to be 

 ample, to prolong the accumulation of the undifferen- 

 tiated cells. In response to this condition, she has 

 instituted for higher types of animals that development 

 which we call embryonic, leaving for the lower types 

 that development which we call larval. 1 Thus we 

 meet in the growth and formation of the higher ani- 

 mals, and in the history of the comparative develop- 



1 The comparison between the larval and embryonic types of development 

 was first made by me in 1895, " Ueber die Vererbung und Verjiingung,'' Bio- 

 logisches Centralblatt., xv., 571-587 ; translation in American Naturalist, xxx., 

 i-g, 89-101. 



