GENERAL BIOLOGY OF EGGS 7 



vitro, but Lewis and Gregory (1929a, b) seem to have been the first 

 to obtain the protracted development of mammalian (rabbit) eggs 

 in culture. 



Role in Animal Economy 



The ovarian egg, as a single cell, has much in common with the 

 other cells of the body, but possesses special features. First distin- 

 guishing traits appear early in embryonic development with the 

 precursor of the egg, the primordial germ cell, which is marked 

 out from the other cells of the embryo by its relatively clear cyto- 

 plasm and large rounded nucleus. This early differentiation has its 

 parallel in phylogeny, for egg-cells, or the equivalents of egg-cells, 

 are recognizable in some of the simplest animals : for example, in 

 members of the Sporozoa, such as the malarial parasite Plasmodium. 

 In certain other unicellular organisms, such as the Trichonympha, one 

 cell bodily enters another, in a manner analogous to the entry of 

 spermatozoon into egg, but the two cells are of much the same 

 size and general appearance (Cleveland, 1958a, b); here there is a 

 functional though not an obvious structural specialization of sex cells. 

 A degree of differentiation of egg-cells is evident, therefore, at least 

 in some members of all the Phyla of the animal kingdom. 



Generally speaking, union of egg and spermatozoon (or of egg- 

 cell and sperm-cell) is followed immediately by a succession of 

 divisions of the resulting zygote, with the formation of a number 

 of new cells, and the process characterizes sexual reproduction. The 

 new cells represent new individuals in unicellular animals, and, 

 adhering together, constitute the embryo in Metazoa. In asexual 

 reproduction, on the other hand, divisions proceed without the 

 occurrence of conjugation or fertilization. Continuity and increase 

 can be maintained in a number of animal populations, particularly 

 in the insect kingdom, by asexual reproduction (see White, 1954), 

 and this fact serves to emphasize that, notwithstanding its close 

 temporal and sometimes causative relationship with cell division, 

 the union of sex cells is not directly concerned with the multiplica- 

 tion of individuals; indeed, its most direct consequence in unicellular 

 organisms is a reduction in number. The capacity for population 

 increase in complex animals depends ultimately upon the poten- 

 tiality for egg production, and the true process of multiplication in 

 mammals is the increase in number of primordial germ cells in the 

 embryonic ovary. The union of the sex cells is primarily of genetic 



