132 EMBRYOLOGY. 



most Vertebrates. Among Articulata, the differences are no 

 less striking, the males being often of a different shape and 

 color, as in crabs, or having even more complete organs, as 

 in many tribes of insects, where the males have wings, while 

 the females are destitute of them, (Fig. 147.) Among mol- 

 lusks, the females have often a wider shell. 



274. Even higher distinctions than specific ones are based 

 upon peculiarities of the sexes ; for example, the whole 

 class of Mammalia is characterized by the fact that the 

 female is furnished with organs for nourishing her young 

 with a peculiar liquid, the milk, secreted by herself. Again, 

 the Marsupial, such as the opossum and kangaroo, are dis- 

 tinguished by the circumstance that the female has a pouch 

 into which the young are received in their immature con- 

 dition at birth. 



275. 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 

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

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

 the embryonic period. 



276. Before the various classes of the animal kingdom 

 had been attentively studied during the, embryonic period, 

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

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

 reptiles, fishes, insects, mollusks, &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 

 sarly changes in the body of the mother. Production from 



