FACTS AND FACTORS OF DEVELOPMENT 19 



there is no protoplasmic connection between 

 'the two. In mammals generally, including 

 also the human species, not a strand of proto- 

 plasm, not a nerve fiber, not a blood vessel 

 passes over from the mother to the embryo; 

 the latter is from the moment of fertilization 

 of the egg a distinct individual with particular 

 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 

 forms in which the eggs are laid before devel- 

 opment begins. 



The fertilized egg of a star-fish, or frog, or 

 man is not a different individual from the 

 adults of these forms, rather it is a star-fish, a 

 frog, or a human being in the one-celled stage, 

 and thereafter this new being maintains its 

 own individuality. This fertilized egg fuses 

 with no other cells, it takes into itself no living 

 substance, but manufactures its own proto- 

 plasm 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 environ- 



