90 ON THE PHYSIOGNOMY OF SERPENTS. 



the terrestrial species devour mainmifera and birds useful 

 to man, and very often destroy the nests to devour the 

 eggs or the young. 



PROPAGATION. 



In our climates, wliere serpents only produced young once 

 a year, copulation takes place most frequently in the first foir 

 days of April or May. For this act the two sexes entwine 

 their bodies together, so as to seem only a single individual 

 with two heads looking face to face ; the male then intro- 

 duces into tlie female cloaca the two cylindric bodies 

 covered \\ith spines, whicli on being turned inside out are 

 drawn from under the tail: the two sexes remain thus imited 

 for several hours ;* but we are unable precisely to fix the 

 duration of their copulation. It is, at least in our indigenous 

 species, a space of three or four months, before the eggs 

 are ready to be laid ; during this interval they undergo a 

 species of incubation in the belly of the mother ; for on 

 opening the eggs just after they are laid, we almost always 

 perceive a foetus more or less developed, and sometimes 

 even perfectly formed. In this latter case, the young are 

 shut up in a thin membrane, which they tear at the moment 

 of birth to commence their independent existence. In a 

 great number of serpents, on the other hand, the eggs are 

 enveloped in a very tenacious tunic of a coriaceous nature, 

 or rather resembling parchment ; the young, being only 

 imperfectly formed when the eggs are laid, they require 

 sometimes the space of a month more before the hatching 

 is accomplished. On this depends the distinction which 

 has been made between A^viparous and ovi23arous serpents : 

 a distinction wiiich, indeed, is not founded on any other 

 ground than a greater or less development of the foetus in 

 the egg at the tune of laj^ng, or on the nature of the ex- 

 terior covermg of the egg. Ophidians are really always ovi- 

 parous, and it is ^^Tong to compare this species of genera- 

 tion to that of the mammifera, where the young receives 

 its nutriment through the medium of the placenta. 



* Lenz, p. 52. 



