EMBRYOLOGY OF CRYPTOBRANCHUS 69 



2. External sexual characteristics 



The adult male may be recognized (Reese, '04) by the presence 

 of a swollen ring about the cloaca, due to glands beneath the skin. 

 This swelling is quite prominent during, and for a few weeks before 

 the breeding season. I found it difficult to distinguish by exter- 

 nal characteristics the sexes of a few specimens taken during the 

 first week of July; during the latter part of July the males could 

 easily be distinguished by the presence of the cloacal swelling. 

 In a few males obtained and examined during the early part of 

 November, the swelling was less pronounced than is usually the 

 case during the breeding season. Females are characterized by 

 the entire absence of the cloacal protuberance found in the male; 

 also, the abdomen of the gravid female is slightly swollen. 



3. Sex ratio and sex segregation 



As a general rule, fewer females than males have been captured. 

 The record of the sex of the great number of adults captured dur- 

 ing the progress of the work is not complete, but the conclusion 

 reached by later work is that the original ratio of 1 : 8 determined 

 (Smith '07) during the fall of 1906 is much too large. In a series 

 of years the proportion of females to males captured is about 1 : 2 

 or 1:3. These results are of course not conclusive as to the actual 

 sex ratio; as will presently be explained, the sex ratio in the speci- 

 mens captured varies for different times and places, and the true 

 ratio may be disguised by the occurrence of seasonal segregation 

 of the females from the more accessible localities. 



In studying the distribution of the sexes throughout the year 

 a distinction must be made between localities which experience 

 has shown are chosen as breeding grounds, and other localities 

 unsuited for breeding purposes. The breeding grounds are char- 

 acterized by shallow water, a moderate current, and the pres- 

 ence of large flat rocks affording cover for cavities protected from 

 the current. Elsewhere a swifter current, smaller rocks barely 

 large enough to serve as cover, or deeper pools of quiet water, 

 afford conditions in which Cryptobranchus can live, but which 

 are not adapted for purposes of reproduction. 



