258 THE THEORY OF THE GENE 



Further study lias shown that the earlier results were 

 obscured by failure to realize that different races of frogs 

 show remarkable differences in the development of the 

 testes and ovaries. Witschi has shown that in general 

 there are two kinds or races of the European grass frog, 

 Rana temporaria. In one of these the testes and ovaries 

 differentiate directly from the early gonad. Such races 

 are found in the mountains and in the far North. In the 

 other races, living in the valleys and in the middle of 

 Europe, the gonad in those individuals that will become 

 males passes through an intermediate stage in which 

 large cells are present in its interior which he regards 

 as immature eggs. These are replaced later by a new 

 set of germ-cells that become the definitive sperm. These 

 races are called undifferentiated races. 



Swingle also finds in the American bullfrog two types 

 or races, speaking broadly, in one of which the testes and 

 the ovaries differentiate early from the progonad. In the 

 other races the differentiation is delayed. In the female 

 of these races the larger cells of the progonad become 

 later the definitive eggs, but in the male the progonad 

 persists for some time after the female has differentiated. 

 Its large cells may differentiate into spermatozoa. These 

 are, however, later absorbed for the most part, but some 

 of those that remain undifferentiated become the defini- 

 tive sperm-cells. Swingle does not interpret the large 

 cells in the male progonad as eggs, but as male sperma- 

 tocytes. He shows that these cells pass through an abor- 

 tive maturation division and then, for the most part, 

 break down. In other words, the male does not pass 

 through a female stage, but makes, as it were, an abortive 

 attempt to form sperm before its second and later differ- 

 entiation takes place. 



Whatever interpretation is placed on these large cells 

 in the progonad, the important point for present con- 



