708 General and Applied Biology 



from body cells or somatoplasm. The continuity of type is thus main- 

 tained through a continuous lineage of germ plasm and germ cells from 

 generation to generation (Fig. 350). 



From the above statements, it is apparent that the germ plasm pre- 

 sents an unbroken descent from generation to generation. The history 

 of this important process is known as the germ plasm cycle and may be 

 divided into the following arbitrarily chosen periods or stages: 



(a) The definite differentiation and segregation in the embryo of 

 one or more primordial germ cells, which are the first cells set aside for 

 the development of future sex cells (Fig. 351 ) . 



'• I #-2. 3. -i 



s • • '■ 



7.---## ## 



mi^. u fim 



1. 



fo/oloio mm %4 Ao/oip^ 





/ V 



II. 



/3. /2 ^^ 



.f ft ft 9; 



If. 



Fig. 350. — Continuity of germ plasm and the origin of somatoplasm. The germ 

 plasm is represented by black and the somatoplasm by white. 1, Sperm from 

 mother's father; 2, egg from mother's mother; 3, sperm from father's father; 4, 

 egg from father's mother; 5, zygote (fertilized egg) from which mother developed; 

 6, zygote (fertilized egg) from which father developed; 7, two cells arising from 

 the zygote by cell division; 8, numerous cell divisions of body cells (somatoplasm), 

 which have arisen from the germ plasm originally; 9, numerous cell divisions of 

 the germ cells (germ plasm) which have arisen from previous germ plasm; 10, 

 egg produced by the mother; //, sperm produced by the father; 12, zygote (fer- 

 tilized egg) of the offspring from which two cells develop by cell division; 13, 

 germ cells (germ plasm) arising originally from the germ plasm of the zygote; 

 14-, body cells (somatoplasm) arising originally from the germ plasm of the zygote. 



