ORGANS OF REPRODUCTION 83 



the snakes provided with claw-like rudiments of 

 hind limbs. The cavities of the hemipenis are 

 connected by a branch with the dorsal artery, and 

 it is by a flow of blood into them that erection of 

 the organ is accomplished. Each hemipenis is 

 lodged in a cavity on each side of the base of the 

 tail; when protruded it turns inside out, and the 

 inner surface becomes the outer, the papillae and 

 erected spines serving to maintain a firm hold 

 in the vagina, from which the organ cannot be 

 withdrawn except by invagination. It has been 

 observed that the presence of spines on the hemi- 

 penis is associated with much tougher vaginal 

 walls. The organ is grooved along its entire 

 length, the groove being the sulcus spermaticus, 

 which, when the edges of the two hemipenes meet, 

 forms with its fellow a canal to convey the semen 

 into the oviduct ; this sulcus may be bifurcate, as in 

 the Viperids and some Colubrids. 



Anal pockets, secretory organs on each side of the 

 vent and lodged in the base of the tail, seem, in 

 females, to be the homologues of the hemipenes ; 

 but this view cannot be held, since the same organs 

 are present, though smaller, in males also, situated 

 dorsally to the hemipenes. The glands with which 

 they are provided produce the strong and offensive 

 odour which appears to be a means of defence in 

 our Grass-snake and other species, and which also 

 serves to bring the sexes together, the glands being 



