SPAWNING HABITS OF CHROSOMUS. I I 



The males possess " pearl organs" (small conical thickenings 

 of the epidermis), occurring over practically the entire surface of 

 the body, head, tail and fins. They are especially numerous and 

 well developed on the dorsal and lateral surfaces of the body, on 

 the opercula, over the entire surface of the tail, and on the dor- 

 sal surfaces of the pectoral fins ; they are large, but comparatively 

 few in number, on the dorsal surface of the head. They are 

 much smaller on the ventral surface of the abdomen, and entirely 

 lacking about the mouth and at the tips of the fins. 



As seen under the microscope, a pearl organ from the body of 

 a male is a sharply pointed transparent spine, rising from a broad 

 base and pointing obliquely backward, occurring as a thickenmg 

 of the epidermis over the middle of the posterior margin of each 

 scale. Usually there is only a single pearl organ for each scale, 

 but sometimes two spines occur close together. Smaller rudi- 

 mentary pearl organs, each consisting of a papilla or thickening 

 of the epidermis without a spine, also occur, one or more over 

 -each scale and usually near its center. 



The pectoral fins of the male are much larger and stronger, in 

 proportion to size of body, than those of the female. As shown 

 by average measurements, the pelvic fins of the male are also 

 proportionally larger than those of the female, but the difference 

 is not so great as in the case of the pectoral fins. 



The females are on the average slightly smaller than the males 

 {see Figs, i and 2), but have the abdomen distinctly swollen. 

 There is a small patch of red just back of the operculum. They 

 have the same yellow coloration as the males, but the color is 

 comparatively faint. They possess only rudimentary pearl organs, 

 hardly visible except under the microscope. 



A notable exception to the usual sexual dimorphism was found 

 in the case of one unusually large female with well developed 

 ovaries and eggs, but with the secondary sexual characteristics 

 of the male all very well marked. 



I had no opportunity to study the color changes and the history 

 of the pearl organs throughout the year. 



Sex Ratio. In order to determine the numerical proportion 



of the sexes, representative samples of several schools were taken 

 on several different days. The tabulated results are as follows : 



