272 



OF THE EGG. 



liar liquid, the milk, secreted by herself. Again, the mar- 

 supialia, such as the opossum and kangaroos, are distinguished 

 by the circumstance that the female has a pouch, into which 

 the young are received in their immature condition at birth. 



433. That all animals are produced from eggs (Omne 

 vivum ex ovo), is an old adage in zoology, which modern 

 researches have fully confirmed. In tracing back the phases 

 of animal life, we invariably arrive at an epoch when the in- 

 cipient animal is enclosed within an egg. It is then called 

 an embryo, and the period passed in this condition is called 

 the embryonic period. 



434. Before the various classes of the animal kingdom 

 had been attentively compared during the embryonic period, 

 all animals were divided into two great divisions : the ovi- 

 parous, comprising those which lay eggs, such as birds, 

 reptiles, fishes, insects, mollusk's, &c., and the viviparous, 

 which bring forth their young alive, like the mammalia, and 

 a few from other orders, as the sharks, vipers, &c. This 

 distinction lost much of its importance when it was shown 

 that viviparous animals are produced from eggs, as well as 

 the oviparous ; only that their eggs, instead of being laid 

 before the development of the embryo begins, undergo their 

 early changes in the body of the mother. Production from 

 eggs should therefore be considered as a universal character- 

 istic of the animal kingdom. 



435. FORM or THE EGG. The general form of the egg is 

 more or less spherical. The eggs of birds have the form of an 

 elongated spheroid, narrow at one end ; and this form is so con- 

 stant, that the term oval has been 

 universally adopted to designate it. 

 But this is by no means the usual 

 form of the eggs of other animals. 

 In most instances, on the contrary, 

 they are spherical, especially among 

 the lower animals. Some have sin- 

 gular appendages, as those of the 

 skates and sharks (fig. 281), which 

 are shaped like a hand-barrow, 

 with four hooked horns at the cor- 

 ners. The eggs of the Hydra, or 

 fresh water polype, are thickly co- 

 vered with prickles (fig, 282). Those of certain insects, for 



Fig. 281. 



Fig. 282. Fig. 283. 



