LIFE-HISTORY OF AMPHIGONOPTERUS AURORA. 



18? 



development of Amphigonopterus and Micrometrus (see next 

 section and Fig. 2). Of 630 embryos of Amphigonopterus aurora 

 (from mothers one to four years old), 337 were found to be 

 males, 293 females (sex-ratio: 100 males to 87 females); the 

 ratio does not vary with the age of the parent fishes, being 100 to 

 86.5 in the embryos from the yearling females only. Of 150 

 embryos of Micrometrus minimus examined, 76 were males, 74 

 females ; the proportion of males to females in each of the seven 

 cases included was, 7 to 9, 9 to 14, 10 to 12, 10 to 13, n to 8, 13 

 to 12, 16 to 6. No tendency toward uniformity in sex even of 

 embryos lying within the same ovarian sheets was evident ; hence 



Live 



Sto 



FIG. 2. Newly-born young male of Amphigonopterus aurora, from near Piedras 

 Blancas, Cal. Dissected to show mature development of testes. 



polyembryony does not occur. Eigenmann recorded similar data 

 for Cymatogaster aggregatus, having distinguished the sexes cyto- 

 logically. 



EARLY DIFFERENTIATION OF THE SEXES, AND THE NATAL 

 MATURITY OF THE MALES. 



Secondary sexual differentiation is early manifest in the de- 

 velopment of Amphigonopterus aurora and Micrometrus mini- 

 mus. The differential number of anal rays characteristic of the 

 sexes of each of these two species (Hubbs, 1918) is clearly ap- 

 parent in embryos only 12 mm. long (the anal rays are first 

 formed when a total length of about 10 mm. has been attained). 

 Eigenmann was unable to distinguish the sexes in Cymatogaster 



