106 COMPARATIVE ANATOMY. 



tive. Such eggs are met with in Aves, Reptilia, cartilaginous 

 fishes, and perhaps also in Cephalopoda and a few other 

 invertebrates. In the third group, the yolk is of medium 

 size and composed of two kinds of granules. But the nutri- 

 tive are but fe.w in number and are subject to considerable 

 variety; the greater part of the mass undergoing segmenta- 

 tion. Within this group are included the eggs of osseous 

 fishes, Batrachia, the higher Crustacea, Arachnida and In- 

 secta. (Thomson.) 



The egg is generally spherical, though occasionally oval, as 

 in Sepia. The shell is commonly smooth, as in Yertebrata, 

 but may present many curious tubercular, fringed or spinous 

 processes, as in Insecta. In Polyzoa the egg is often fur- 

 nished with bifid hooks, as in Plumatella. 



The investing membrane of egg is termed the chorion. 

 It may be shaggy, as in Mammalia, or smooth, as in all 

 others. In eggs of the second group the chorion (membrana 

 putaminis) is divided into two layers, the outer one lining 

 the shell, the inner one enclosing the albumen (white of egg). 

 At the ' butt' of the egg the two layers are wide apart, con- 

 stituting the 'air chamber.' Within the albumen a chala- 

 ziferous membrane is found forming the 'chalaza' upon 

 either pole of the vitellus. Immediately surrounding the 

 vitellus is a diaphanous vitelline membrane. The ' egg shell' 

 may be composed of carbonate of lime, as in certain Entozoa, 

 Helix, and oviparous vertebrates ; or one of various kinds of 

 coriaceous and chitinous structures. 



The number of eggs varies greatly in different animals. As 

 a rule, it bears a direct proportion to the risks attending de- 

 velopment. Where the ova are well protected during gesta- 

 tion, as in the placental mammal, a few only escape at a time, 

 and with many animals, as the human female, the number 

 is restricted to one. With aquatic forms, on the other hand, 

 the eggs may lie unprotected during gestation. Great num- 

 bers are, in consequence, consumed by carnivorous animals, 

 become infected with parasites, or may otherwise fail to attain 



