THE DUALITY OF INHEBITANCE 



represented in part by the conspicuous oil- 

 drops seen in the egg (the heavy circles in 

 Fig. 2). 



The egg of a mouse needs no such store of 

 nourishment, since in common with the young 

 of other mammals the mouse-embryo nourishes 

 itself by osmosis from the body fluids of the 

 mother. The mouse-egg is accordingly smaller. 

 Stages in its fertilization are shown in Fig. 4. 

 In A the sperm has already entered the egg. 

 Remnants of its thread-like tail may still be 

 seen there. Nearby is seen a nuclear body 

 derived from the sperm-head. Opposite is 

 seen the nuclear body furnished by the egg 

 itself. The two nuclear bodies fuse and their 

 united substance is then distributed to all 

 parts of the embryo-mouse, just as happens in 

 the development of the worm, Nereis. 



There are reasons for thinking that the 

 nuclear material is especially important in re- 

 lation to heredity and that the equal share of 

 the two parents in contributing it to the em- 

 bryo is not without significance, for inheritance, 

 as we have seen, is from both parents in equal 

 measure. In cases where the inheritance from 



13 



