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 considering the manner in which impregnation 

 takes place, it is necessary to give a general view of the 

 structure of the egg, employing that of the hen as an ex- 

 ample. The shell or external covering, consists, according 

 to VAUQUELTN, 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 a cavity, filled with atmosphe- 

 ric air, termed jblliculus aeris, 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 



