PROPAGATION. 
91 
The necessary conditions for the development of the 
Emhryon in the egg 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 favourable to evaporation and absorption 
through the exterior covering of the egg. 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 laevis, for example, producer 
living young, as our common viper ; but several other Cor- 
onellse 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, j* 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 wholly 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 
^ Oversigty 183, p. 4. t Lei^z p. 498. 
