THE GESTATION PERIOD IN THAMNO- 

 PHIS BUTLERII (COPE), 



In 1912 the writer (Biol. Bui., XXIV, pp. 18- 



20) published the results of observations on the 

 breeding habits of Thamnophis butlerii (Cope), some 

 of which concerned the length of the period of gesta- 

 tion in southern Michigan. Since the snakes have 

 been observed to breed at various times between April 

 9 and April 24, and the time of breeding is probably 

 determined largely by the prevailing temperatures 

 during April, and because the young appear in Au- 

 gust or early in September (August 7-September 

 6), it was concluded that "it is very probable that the 

 length of the gestation period is rather exact for the 

 species," the differences in the time of parturition 

 being largely due to differences in the time of breed- 

 ing. It is to be expected that the period is subject 

 to some variation, since presumably the body tem- 

 perature of the embryo, like that of the mother, fluc- 

 tuates with the temperature of the air, and this influ- 

 ences the rate of development. 



The records which have been obtained show that 

 the length of the period may vary as much as 40 days. 

 The female which was under observation in 1912, gave 

 birth to young in 144 days, but a female which was 

 fertilized on April 18, 1913, had her young on July 

 81, or in 104 days, and in the case of one which be- 

 came pregnant on April 18, 1914, parturition oc- 

 curred on August 9, or in 113 days. 



The factors which cause this variation have not 

 been determined, but that temperature is one is sug- 

 gested by the fact that the snakes kept in relatively 

 cool places during gestation, had the longest periods. 

 The female which gave birth in 144 days was kept in 

 a cool place in the laboratory and was not exposed to 

 sunlight, while the female with the next shorter pe- 

 riod was kept in a warm place and exposed to the 

 sunlight for a short time each day, and the female 

 which had young in 104 days was kept in a glass- 

 covered cage in a warm room where the sunlight fell 

 upon it for several hours each day. Unfortunately, 

 the females which had copulated and were kept as 

 controls, proved to be sterile, and the observations are 



