GERM-CELL ORIGIN 117 



of the nucleus as a carrier of the hereditary substances or genes and a pecuHar, 

 speciaHzed, germinal cytoplasm. The character of the cytoplasm of the germ 

 cell is the main factor distinguishing a germ cell from other soma cells. 



The matter of a germinal cytoplasm suggests the necessity for a segregation 

 of the germinal plasm in the form of specific germ cells during the early 

 development of the new individual. As a result, great interest, as well as con- 

 troversy, has accumulated concerning this aspect of the germ-cell problem: 

 namely, is there a separate germinal plasm set apart in the early embryo which 

 later gives origin to the primordial germ cells, and the latter, after migration 

 (fig. 60), to the definitive gonia; or according to an alternative view, do some 

 or all of the definitive germ cells arise from differentiated or relatively undif- 

 ferentiated soma cells? The phrase primary primordial germ cells often is 

 used to refer to those germ cells which possibly segregate early in the embryo, 

 and the term secondary primordial germ cells is employed occasionally to 

 designate those which may arise later in development. 



The dispute regarding an early origin or segregation of the germinal plasm 

 in the vertebrates also occurs relative to their origin in certain invertebrate 

 groups, particularly in the Coelenterata and the Annelida (Berrill and Liu, 

 '48). In other Invertebrala, such as the dipterous insects and in the ascarid 

 worms, the case for an early segregation is beyond argument. An actual dem- 

 onstration of the continuity of the Keimbahn from generation to generation 

 is found in Ascaris megalocephala described by Boveri in 1887. (See Hegnc, 

 '14, Chap. 6.) In this form the chromatin of the somatic cells of the body 

 undergoes a diminution and fragmentation, whereas the stem cells, from which 

 the germ cells are ultimately segregated at the 16-cell to 32-cell stage, retain 

 the full complement of chromatin material (fig. 61). Thus, one cell of the 

 16-cell stage retains the intact chromosomes and becomes the progenitor of 

 the germ cells. The other 15 cells will develop the somatic tissues of the body. 

 The diminution of the chromatin material in this particular species has been 

 shown to be dependent upon a certain cytoplasmic substance (King and 

 Beams, '38). 



In some insects the Keimbahn also can be demonstrated from the earliest 

 stages of embryonic development. In these forms a peculiar polar plasm within 

 the egg containing the so-called "Keimbahn determinants" (Hegner, '14, 

 Chap. 5) always passes into the primordial germ cells. That is, the ultimate 

 formation and segregation of the primordial germ cells are the result of nuclear 

 migration into this polar plasm and the later formation of definite cells from 

 this plasm (fig. 62). The cells containing this polar plasm are destined thus 

 to be germ cells, for they later migrate into the site of the developing germ 

 glands and give origin to the definitive germ cells. 



Many investigators of the problem of germ-cell origin in the vertebrate 

 group of animals have, after careful histological observation, described the 

 germ cells as taking their origin from among the early entodermal cells (see 



