114 THE DEVELOPMENT OF THE GAMETES OR SEX CELLS 



the male, these undifferentiated sex cells are called spermatogonia; if female, 

 they are known as oogonia. 



B. Controversy Regarding Germ-cell Origin 



The problems of germ-cell origin in the individual organism and of the 

 continuity of the germ plasm from one generation to the next have long 

 been matters of controversy. Great interest in these problems was aroused 

 by the ideas set forth by Waldeyer, Nussbaum, and Weismann during the 

 latter part of the nineteenth century. Waldeyer, 1870, as a result of his studies 

 on the chick, presented the "germinal epithelium" hypothesis, which main- 

 tains that the germ cells arise from the coelomic epithelium covering the 

 gonad. Nussbaum, 1880, championed the concept of the extra-gonadal origin 

 of the germ cells. According to this view, derived from his studies on frog 

 and trout development, the germ cells arise at an early period of embryonic 

 development outside the germ-gland area and migrate to the site and into 

 the substance of the germ gland. 



At about this time the speculative writings of August Weismann aroused 

 great interest. In 1885 and 1892 Weismann rejected the popular Darwinian 

 theory of pangenesis, which held that representative heredity particles or 

 "gemmules" passed from the body cells (i.e., soma cells) to the germ cells 

 and were there stored in the germ cells to develop in the next generation 

 (Weismann, 1893). In contrast to this hypothesis he emphasized a complete 

 independence of the germ plasm from the somatoplasm. He further suggested 

 that the soma did not produce the germ plasm as implied in the pangenesis 

 theory, but, on the contrary, the soma resulted from a differentiation of the 

 germ plasm. 



According to the Weismannian view, the germ plasm is localized in the 

 chromosomal material of the nucleus. During development this germ plasm 

 is segregated qualitatively during successive cell divisions with the result that 

 the cells of different organs possess different determiners. However, the nuclear 

 germ plasm (Keimplasma) is not so dispersed or segregated in those cells 

 which are to become the primitive sex cells; they receive the full complement 

 of the hereditary determiners for the various cells and organs characteristic 

 of the species. Thus, it did not matter whether the germ cells were segregated 

 early in development or later, so long as the nucleus containing all of the 

 determinants for the species was kept intact. In this manner the germ plasm, 

 an immortal substance, passed from one generation to the next via the nuclear 

 germ plasm of the sex or germ cells. This continuity of the nuclear germ 

 plasm from the egg to the adult individual and from thence through the 

 germ cells to the fertilized egg of the next generation, constituted the Weismann 

 "Keimbahn" or germ-track theory. The soma or body of any particular gen- 

 eration is thus the "trustee" for the germ plasm of future generations. 



The Weismannian idea, relative to the qualitative segregation of the chro- 



