282 



A TEXTBOOK OF BOTANY 



[Ch. VI, 4 



very first division of the fertilized egg cell its nucleus dis- 

 plays a number of chromosomes precisely equal to that which 

 we know the sperm and egg nuclei possess together, while 

 in some cases it is found possible to identify the two sets. 

 All evidence seems to indicate that this bringing together 

 of the chromosomes (or chromatin) from two parent cells 

 into a single nucleus is the principal (and perhaps only) 

 functional end of fertilization. The accompanying com- 

 mingling of the cytoplasm appears not to be important, 

 and indeed in some cases seems not to occur, only the nucleus 

 of the sperm cell passing into the egg cell. 



Thus we are brought to ask what may be the significance 



Fig. 194. — Diagram to illustrate the performance of the chromosomes 

 in the fusion of the germ cells. 



The two chromosomes from the pollen or male parent are white, and those 

 from the egg cell or female parent are black. 



of the chromosomes. Our knowledge of them is still im- 

 perfect, but this much seems clear, — that they are the phys- 

 ical basis of heredity, the carriers of the factors which cause 

 the new individual to develop in general like its parents. 

 The chromosomes do not themselves build the cells out of 

 which the organism is constructed, for that is clearly done 

 by the cytoplasm; but in some way, still unknown, the 

 constructive work of the cytoplasm seems guided by the 

 chromosomes, which thus must contain the plans, so to 

 speak, of the new structure. Furthermore, and this is im- 

 portant, the evidence indicates that the set of chromosomes 

 contributed by each nucleus in fertilization is complete, that 

 is, capable of guiding the construction of a complete organism 



