PROrAGATION. 91 



The necessary conditions for the development of the 

 Emhryon in the agg are, according to M. Herholdt,* the 

 humidity produced by a feeble vegetable fermentation con- 

 joined to a moderate temperature, amounting to between 

 + 20 and + 6 R. (from 77° to 46 F.) ; and finally, 

 circumstances favoru-able to evaporation and absorption 

 through the exterior covering of the Qgg. Hence serpents 

 seek to deposit their eggs in places where these conditions 

 are combined, as in a dunghill, or a mass of leaves collected 

 in places exposed to the sun ; it is the same reason that 

 induces many oviparous species to establish themselves in 

 the vicinity of houses, or conservatories. 



It has been erroneously advanced that venomous ser- 

 pents are always viviparous, and that serpents not venom- 

 ous alone lay eggs : it is not so ; for many of the latter are 

 viviparous, while certain species of the former tribe lay eggs, 

 like the majority of the Colubri. It even appears that this 

 diversity in the generation has no relation to the organiza- 

 tion of the animal itself ; for these two modes of reproduc- 

 tion are sometimes observed in nearly allied species of the 

 same genus ; the Coronella Igevis, for example, produces 

 li\dng young, as our common viper ; but several other Cor- 

 onella3 lay eggs inclosed in a coriaceous envelop ; the same 

 is the case with the Python bivittatus, whilst the Boa 

 murina is completely viviparous : among the venomous 

 snakes, the Najas, and several others are oviparous. 



During the laying, serpents keep themselves stretched 

 on the ground, and only lift the tail to permit the eggs to 

 escape ; this operation is neither long nor painful, t The 

 eggs of Ophidians contain, before the development of the 

 embryon, a homogeneous fluid of a deep yellow colour, ana- 

 logous to the yolk of the eggs of birds. The white fluid 

 appears to be whoUy wanting in the eggs of serpents : they 

 are also distinguished from those of birds by the total want 

 of the air vesicle. The yolk is covered by a proper tunic, 

 provided with numerous bloodvessels, the principle trunks 

 of which unite with the canal of the vitellus at the umbi-^ 

 licus of the embryon : this membrane, named Allantoid 



^ Oversigt, 183, p. 4. t Lexz p. 498. 



