OF THE EGG. 103 



Among Articulata, the differences are no less striking, the 

 male being often of a different shape or 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 

 deprived of them. 



274. Even higher than specific distinctions 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 order Marsupialia, to which the opossum belongs, is dis- 

 tinguished by the circumstance of the female having a 

 pouch in which the young are received after 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 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, insects, mollusks, dec., and the viviparous, which 

 bring forth their young alive, namely, the mammalia. 

 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 

 characteristic of the Animal Kingdom. 



277. Form of the Egg. The general form of the egg 



