422 



The Journal of Heredity 



Carriers of the Germ Plasm 



The two large spheres in the top 

 picture are egg cells, alive in Ringer's 

 solution when photographed. A single 

 whip-like spermatozoon may be seen 

 at the point of the arrow. The pene- 

 tration of an egg cell by a single sper- 

 matozoon results in a cell, the fertilized 

 egg, which has within itself the power 

 of developing into a new individual 

 when properly nourished. 



The middle picture shows an egg 

 cell which has been penetrated by a 

 spermatozoon. The unabsorbed tail 

 of the latter can be seen next to the 

 single nucleus produced by the fusion 

 of the egg and sperm nuclei. 



A living egg in the two-cell stage 

 appears in the illustration at the bot- 

 tom. Under normal conditions the 

 two cells into which the egg cell 

 divides after fertilization remain to- 

 gether and become the millions of cells 

 which form a single individual. After 

 the early cell divisions, however, each 

 of the cells still possesses the poten- 

 tiality for development into a perfect 

 individual, and may do so if separated. 

 Human identical twins are produced 

 by the breaking apart of a single egg 

 in the two-cell stage, or very soon 

 after, and resemble each other closely 

 because of their identical heredity. 

 Ordinary twins develop from two 

 wholly independent egg cells which 

 merely happen to be fertilized at about 

 the same time. Such twins resemble 

 each other no more than ordinary 

 brothers and sisters. These photo- 

 micrographs, of white rat eggs cells 

 magnified 500 diameters, are by Dr. 

 Joseph Long, of the University of 

 California. 



Fig. 21 



