THE DUALITY OF INHEEITANCE 



and sperm unite, in what we call the fertiliza- 

 tion of the egg. The egg is a rounded body 

 incapable of motion, but the sperm is a minute 

 thread-like body which moves like a tadpole 

 by vibrations of its tail. In the case of most 

 animals which live in the water, egg and sperm- 

 cells are discharged into the water and there 

 unite and develop into a new individual, but 

 in the case of most land animals this union takes 

 place within the body of the mother. We may 

 consider an illustration of either sort. 



The fertilization of the egg of a marine 

 worm, Nereis, is shown in Fig. 2. The thread- 

 like sperm penetrates into the egg. Its en- 

 larged head-end forms there a small nuclear 

 body, which increases in size until it equals 

 that of the egg-nucleus, with which it then fuses. 

 The egg next begins to divide up to form the 

 different parts of a new worm-embryo. To 

 each of these parts the nuclear material of egg 

 and sperm is distributed equally. Since this 

 development takes place wholly outside the 

 body of either parent it is necessary that the 

 egg contain enough food to last until the young 

 worm can feed itself. This food material is 



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