FACTS AND FACTORS OF DEVELOPMENT 



5 2 9 



maintains its own individuality. This fertilized egg fuses with no other 

 cells, it takes into itself no living substance from without, but manu- 

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

 living thing, an individuality. Under proper environmental conditions 

 this fertilized egg cell develops, step by step, without the addition of 

 anything from without except food, water, oxygen, and such other raw 

 materials as are necessary to the life of any adult animal, into the im- 

 mensely 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 develops, step by step, without the addition of anything from with- 

 out except raw materials and environmental stimuli, the multifarious 



1 st PB. 









9 



Egg of Nereis, less highly magnified than Figs. 4 and 5, showing the progress of the 

 turation spindle of the egg (1st Mat. Sp.) ; Fig. 7, the first polar body (1st PB) 

 sperm nucleus and spindle (£2V) ; Fig. 9, the division of the male and female nuclei 



