300 GENERAL BIOLOGY 



the new organism must differ in composition from the old. 



Every organism, therefore, that is developed from a fer- 

 tilized egg sets out in life with a material endowment that is 

 derived from two antecedent cells. In its nuclear equip- 

 ment there are two more or less unlike sets of chromosomes. 

 It is probable that, by the precise mitotic method, the sub- 

 stance of both paternal and maternal chromosomes (fig. 178) 

 is equally divided and distributed at every cell division. 

 We can see that this is so, when paternal and maternal 

 chromosomes are visibly different in form, as is notably the 

 case in certain species that may be hybridized; for in the 

 hybrid embryos two sorts of chromosomes reappear, con- 

 stant in number and form and grouped by themselves, in 

 successive cell divisions. 



Chromosomes. — -Protoplasm, the physical basis of life, is 

 of course, the material basis of heredity. Among proto- 

 plasmic structures, those of the nucleus maintain the great- 

 est permanence and uniformity of behavior. The chromo- 

 somes especially give evidence of continuing individuality of 

 organization. What the chromosomes are we do not know. 

 That they are chemical substances is indicated by their 

 micro-chemical reactions; it is by means of their reactions 

 to specific stains that we are able to bring them clearly into 

 view. Their vital organization is complex. That they 

 play an important role in cell division is sufficiently obvious ; 

 mitosis might well have for its object the equitable division 

 of them among the descendent cells. That their role in 

 sexual reproduction is likewise important is indicated by 

 their uniform and parallel behavior in egg and sperm while 

 cytoplasmic parts are undergoing the greatest differentia- 

 tion. 



Naturally, the greatest speculative interest has centered 

 about the chromosomes. They have been assumed to be 

 the bearers of hereditary characters, and the agents of 



