416 PHILOSOPHY OF ZOOLOGY. 
as in the drake, and many species of the inferior classes ; 
at the base of a solid penis, along which there is a groove, 
as in the ostrich, or in one or two tubercles, situated in the 
cloaca, the common opening of the ureters and rectum. 
The influence of the season of love is much more con- 
siderable on the male organs of oviparous than viviparous 
animals. They increase to an extraordinary size, appear 
full of sperm, and occupy a large portion of the cavity of 
the abdomen. At other seasons they diminish so much, 
as in some cases to be detected with difficulty. 
1. Female organs.—Among the oviparous animals, the 
female organs exhibit very remarkable differences in their 
structure and functions. In birds the ovarium is single, 
while in fishes it is double. The oviduct either leads to 
the cloaca directly, or suffers a previous uterine enlarge- 
ment, or it terminates in a tubular elongation, to which the 
name of ovipositor has been applied. 
Before considermg the manner in which impregnation 
takes place, it 1s necessary to give a general view of the 
structure of the ege, employing that of the hen as an ex- 
ample. The shell or external covering, consists, according 
to VauauELty, of 89.6 of carbonat of lime, 5.7 of phos- 
phat of lime, and 4.7 of animal matter. It is penetrated 
by numerous pores, through which air passes during incu- 
bation. On its inner surface, and as an integument to the 
remaining contents, is found the firm, white membrane 
termed membrana albuminis. At the larger end of the 
egg, this membrane includes acavity, filled with atmosphe- 
ric air, termed folliculus aéris, which, by degrees, enlarges 
during incubation. ‘The membrane includes the glaire or 
white, divisible into two layers, each surrounded by a deli- 
cate membrane. The external layer is the most fluid and 
transparent. The centre of the egg is occupied with the 
yolk, enveloped by its peculiar membrane, termed the 
