CELL DIVISION AND GAMETOGENESIS 



225 



with the nucleus, dividing in two by simple constriction without 

 the accompaniment of chromosome and spindle formation 



(Fig. 138). 



Gametogenesis. — Reproduction in Protozoa may consist 

 simply in the division of the animal as a whole into two parts, 

 each of which then grows to the size of the original. In sexual 

 reproduction among Metazoa, the egg and sperm (spermatozoon) 

 are cells that have the power, when united in fertilization, of 

 developing into a new organism by cell division, accompanied by 

 growth and differentiation. Some Protozoa also produce germ 

 cells and exhibit a primitive form of sexual reproduction. The 

 asexual reproduction that occurs in some Metazoa does not 

 involve the production of germ cells. 



Fig. 138. — Amitosis in nurse cells of ovary of the potato beetle, Leptinotarsa 



signaticollis. 



All of the cells of the metazoan body develop from the fertilized 

 egg. That means that both somatic and germ cells have a com- 

 mon origin in the fertilized egg. The essential fact in fertiliza- 

 tion, to be considered more in detail later, is the formation of a 

 single cell, the fertilized egg, from the fusion of the egg nucleus 

 with a nucleus brought in by the sperm. The number of chromo- 

 somes, the zygotic or diploid number, occurring in the somatic 

 cells of the animal is thus determined at fertilization. The egg 

 then undergoes cell division (cleavage) by the mitotic method, as 

 described in the preceding pages, the cells or blastomeres thus 

 produced containing the potencies of both somatic and germ 

 cells. Then, after a number of cell generations have been 

 produced, certain cells, called primordial germ cells, may be 

 distinguished from the somatic cells. There is evidence in 

 many animals that the appearance of the primordial germ cells 

 marks an important and final differentiation of germ-cell material 

 from somatic. In higher forms, it is generally thought that the 

 functional germ cells are the descendants of the primordial germ 



