GENERAL ZOOLOGY 



Fig. 5.5. The oviducts of mammals, showing progressive fusion of uterine portions. A, duplex 

 uterus, as in rabbits; B, bicornuate uterus, as in swine; and C, simplex uterus, as in man. 



all the somatic cells, that is, from the zygote, the single cell from which the 

 entire organism develops (p. 145). In some animals it is possible to trace 

 through successive cell divisions the history of the germ cells, which may be 

 set aside at a very early stage of development (Fig. 5.6). During the time 

 when the somatic cells are increasing in number and becoming specialized, 

 the potential germ cells become localized in the reproductive organs in an 

 undifferentiated condition. Usually rounded, the cytosome is relatively small 

 in comparison with the nucleus. There is little activity until the animal 

 nears the age of sexual maturity. 



Specialization of the germ cells, or gametes, occurs in vertebrates during the 

 process of gametogenesis, or maturation. The maturation or differentiation of 

 a male germ cell is spermatogenesis, the origin of the spermatozoon, or sperm; 

 maturation of a female germ cell is oogenesis, the origin of the ovum, or egg. 



2nd polar body 

 Blastula cavity 



Primordial germ cell 



Fig. 5.6. Origin of primordial germ cells. A, section of a late blastula stage in the de^^elop- 

 ment of a mollusk, Sphaenum slnatinum: x265. B, section of a blastula stage (32 cells) of a 

 roundworm, Parascam equnrum {Ascaris megalocephala bivalens): x710. {A, from F. H. 

 Woods, 1931, Journal of Morphology and Physiology, vol. 51.) 



134 



