FACTS AND FACTORS OF DEVELOPMENT 21 



carbonic acid and other waste products; it is 

 sensitive to certain alterations in the environ- 

 ment such as thermal, chemical and electrical 

 changes — it is, in short, a distinct living thing, 

 an individuality. Under proper environ- 

 mental conditions this fertilized egg cell de- 

 velops, step by step, without 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 activities, reactions, 



one sperm has penetrated the jelly layer (jl) which surrounds 

 the egg and the peripheral protoplasm (pp) of the egg pro- 

 trudes as an entrance cone (ec) to meet it. (After Fol.) 

 D, Mature spermatozoon of the sea-urchin, Toxopneustes, 

 showing head (h) ; middle-piece (m) ; and tail t). E-H, Suc- 

 cessive stages in the penetration of the sperm nucleus (c?IV) 

 and centrosome (c$C) into the egg of Toxopneustes. I-L, 

 Stages in the approach of the sperm nucleus (c?IV) to the egg 

 nucleus ($IV), and in the division of the sperm centrosome 

 (<$€') and the formation of the first cleavage spindle. (D-L 

 after Wilson.) 



