120 THE DEVELOPMENT OF THE GAMETES OR SEX CELLS 



know what injurious effects the x-rays may produce upon the ability of the 

 various cells of the germinal epithelium to differentiate. 



An experimental study of the early, developing, amphibian embryo relative 

 to the origin of the primordial germ cells also has been made by various in- 

 vestigators. Bounoure ('39) applied a vital-staining technic to certain anuran 

 embryos. The results indicate that the germinal plasm in these forms is asso- 

 ciated with the early, entodermal, organ-forming area located at the vegetal 

 pole of the cleaving egg. This germinal plasm later becomes segregated into 

 definite cells which are associated with the primitive entoderm. At a later 

 period these cells migrate into the developing germ gland or gonad. On the 

 other hand, experimental studies of the urodele embryo indicate that the 

 early germinal plasm is associated with the mesoderm (Humphrey, '25, '27; 

 Nieuwkoop, '49 ) . Existence of an early germinal plasm associated with the 

 entoderm in the Anura and with the mesoderm in the Urodela thus appears 

 to be well established for the amphibia. 



The evidence derived from amphibian studies together with the observa- 

 tions upon the fish group presented in the table (see pp. 121-124) strongly 

 suggests that an early segregation of a germinal plasm (germ cells) occurs in 

 these two major vertebrate groups. Also, in birds, the experimental evidence 

 presented by Benoit ('30), Goldsmith ('35), and Willier ('37) weighs the 

 balance toward the conclusion that there is an early segregation of germ cells 

 from the entoderm. Similar conditions presumably are present in reptiles. In 

 many vertebrates, therefore, an early segregation of primordial germ cells and 

 their ultimate migration by: (1) active ameboid movement, (2) by the 

 shifting of tissues, or (3) through the blood stream (see table, pp. 121-124) 

 to the site of the developing gonad appears to be well substantiated. 



The question relative to the origin of the definitive ova in the mammalian 

 ovary is still in a confused state as indicated by the evidence presented above 

 and in the table on pp. 121-124. Much more evidence is needed before one 

 can rule out the probability that the primordial germ cells are the progenitors 

 of the definitive germ cells in the mammals. To admit the early origin of pri- 

 mordial germ cells on the one hand, and to maintain that they later disappear 

 to be replaced by a secondary origin of primitive germ cells from the germinal 

 epithelium has little merit unless one can disprove the following position, to 

 wit: that, while some of the primordial germ cells undoubtedly do degenerate, 

 others divide into smaller cells which become sequestered within or imme- 

 diately below the germinal epithelium of the ovary and within the germinal 

 epithelium of the seminiferous tubules of the testis, where they give origin 

 by division to other gonial cells. Ultimately some of these primitive gonia 

 pass on to become definitive germ cells. 



However, aside from the controversy whether or not the primordial germ 

 cells give origin to definitive germ cells, another aspect of the germ-cell 

 problem emphasizing the importance of the primitive germ cells is posed by 



