r-,2 



Heredity and Eugeiiies 



cytoplasm is as essential to the process as the nucleus; but 

 in certain plants, notably Lilium, it has been demonstrated 

 that when fusion occurs there is no cx'toplasm whatsoever 

 investing the male nucleus. It seems safe to conclude, 

 therefore, that the nucleus contains the material essential 

 to the phenomena of heredity; and if so, chromatin 

 must be the material, and the chromosomes its visible 

 organized units. 



A very simple case will serve to illustrate the results of 

 the sexual fusion upon the chromosome situation. Imagine 



the fusion of an egg and a 

 sperm, each of whose nuclei 

 contains two chromosomes 

 (Fig. 5, I and 2). The nu- 

 cleus of the fertilized egg 

 would contain four chromo- 

 somes (Fig. 5, 3), two of 

 them maternal (contributed 

 by the egg), and two of 

 them paternal (contributed 

 by the sperm). Suppose 

 that two of the four domi- 

 nate in determining the 

 structure of the new indi- 

 vidual to be developed from 

 the fertilized egg. It will 

 be seen that there are four 

 possible pairs: (i) the paternal pair (Fig. 5, 4), in which 

 case the new individuals would resemble the male parent; 

 (2) the maternal pair (P'ig. 5, 7), in which case the resem- 

 blance would be to the female parent; (3) two pairs (Fig. 5, 5 

 and (5), each consisting of a dominant male and a dominant 



Fig. 5. — Diagram illustrating result 

 of sexual fusion: j, sperm containing two 

 chromosomes {AA)\ 2. egg containing 

 two chromosomes (BB) ; j, fertilized egg 

 containing four chromosomes (two pa- 

 ternal and two maternal); 4, domination 

 of paternal ihrumosomcs; 5 and (5, domi- 

 nation of iiiu- [jaternal and one maternal 

 chromosome; 7, domination of maternal 

 chromosomes. 



