INHERITANCE 305 



to develop by the experimental addition of sperm cells (fig. 

 207); the sperm cell enters as it would to fertilize the egg 

 nucleus, but instead, takes the place of that nucleus, and 

 then development proceeds. Evidently the necessary 

 nuclear equipment for development is present in the sperm 

 as well as in the egg, and is duplicated in the zygote in cross 

 fertilization. 



The chief facts now before us, regarding the material basis 

 for inheritance are : 



1. The continuity of the germ plasm through the genera- 

 tions. 



2. In cell division, mitosis, a process apparently well 

 fitted for carrying development forward along the even 

 tenor of its way. 



3. In sexual reproduction, fertilization, a process ap- 

 parently well fitted for introducing new elements into cell 

 lineage. 



4. Preliminary to fertilization, synapsis and chromosome 

 reduction. 



5. The development of eggs without fertilization in cases 

 of parthenogenesis. 



6. The duplication of the chromosome content of the 

 nucleus in fertilization. 



Study jy. Observations on cell division, and on the matur- 

 ation of sex cells. 



Materials needed: Prepared slides showing clearly the 

 chief phenomena of cell division, either in growing tissues or 

 in developing egg cells. Freshly laid and living eggs of pond 

 snails showing polar bodies. 



The student, duly cautioned as to the damage wrought in 

 tissues in section making, and expedited somewhat in his 

 observations by the guidance of the teacher (a demonstra- 

 tion with projection microscope will for these purposes be 



