266 THE THEORY OF THE GENE 



the sex of the offspring produced by these transformed 

 females, although ripe sperm has been found in one case 

 at least. Recently Essenberg has studied the development 

 of the gonads in the young of this fish. At birth the fish 

 measures 8 mm. and the gonads are in an '^ indifferent 

 stage," containing two kinds of cells of peritoneal origin. 

 At 10 mm. the sexes are distinct; in the females the pri- 

 mordial germ-cells gradually change into young eggs ; in 

 the male the definitive germ-cells (sperm-cells) come 

 from the peritoneal cells. In the immature condition, be- 

 tween 10 and 26 mm. in length, Essenberg records 74 

 females and 36 males, counting amongst the females the 

 retrogressive types, i.e., those in process of transforma- 

 tion from "females" to "males." The sex ratio of adult 

 fish, taken from Bellamy's records, is given as 75 5 to 

 25 9 . The change does not appear to be due to differential 

 viability, but to "sex inversion." This occurs most com- 

 monly in fishes from 16 to 27 mm., but may occur in later 

 stages also. The data indicate, then, that approximately 

 half of the "females" become males. This statement does 

 not mean, however, that functional females have changed 

 into males, but that half of the young "females" are 

 identified as such by the presence of an ovary, which 

 later changes into a testis. Recently Harms (1926) 

 records in Xiphophorus the change of old females, that 

 have become sterile, into functional males. These trans- 

 formed females when bred as males produced only fe- 

 male offspring which means if this fish is homogametic 

 that all the functional sperms of the secondary male are 

 X-bearing. 



A curious case has recently been described by Junker 

 in the stonefly, Perla marginata. The young males (Fig. 

 143) pass through a stage in which an ovary is present 

 that contains rudimentary eggs (Fig. 143). The male has 

 an X- and a Y-chromosome and the female two X's (Fig. 



