I'm ts and Factors of Development 15 



the new individual is always distinguishable from the body of the 

 mother since- there is no protoplasmic connection between the two. 

 In mammals generally, including also the human species, not a 

 strand of protoplasm, not a nerve fiber, not a blood vessel passes 

 over from the mother to the embryo; the latter is from the mo- 

 ment of fertilization onward a distinct individual with particu- 

 lar individual characteristics, and this is just as true of viviparous 

 animals in which the egg undergoes a part of its development 

 within the body of the mother as it is of oviparous ones in which 

 the eggs are laid before development begins. 



The fertilized egg of a star-fish or frog or man is not a dif- 

 ferent individual from the adult form into which it develops, 

 rather it is a starfish, a frog, or a human being in the one-celled 

 stage. This fertilized egg fuses with no other cells, it takes into 

 itself no living substance, but manufactures its own protoplasm 

 from food substances ; it receives food and oxygen from without 

 and it gives out carbonic acid and other waste products ; it is 

 sensitive to certain alterations in the environment such as ther- 

 mal, chemical and electrical changes — it is, in short, a distinct 

 living thing, an individual or person. Under proper environmen- 

 tal conditions this fertilized egg cell develops, step by step, with- 

 out the addition of anything from the outside except food, water, 

 oxygen, and such other raw materials as are necessary to the life 

 of any adult animal, into the immensely complex body of a star- 

 fish, a frog, or a man. At the same time, from the relatively 

 simple reactions and activities of the fertilized egg there develop, 

 step by step, without the addition of anything from without except 

 raw materials and environmental stimuli, the multifarious activi- 

 ties, reactions, instincts, habits, and intelligence of the mature 

 animal. 



Is not this miracle of development more wonderful than any 

 possible miracle of creation ? And yet as one watches this mar- 

 vellous process by which the fertilized egg grows into the embryo, 

 and this into the adult, each step appears relatively simple, each 

 perceptible change is minute ; but the changes are innumerable and 



