CHAPTER TENTH. 



EMBRYOLOGY. 

 SECTION I. 



OF THE EGG. 



429. THE functions of vegetative life, of which we have 

 treated in the preceding chapters, namely, digestion, circula- 

 tion, respiration and secretion, have for their end the preser- 

 vation of the individual. We have now to treat of the 

 functions that serve for the perpetuation of the species, 

 namely those of reproduction ( 308). 



430. It is a law of nature that animals as well as plants 

 are the offspring of individuals of the same kind, and vice 

 versa, that none of them can give birth to individuals differing 

 from themselves ; but recent investigations have modified to a 

 considerable extent this view, as we shall hereafter see. 



431 . Reproduction in animals is almost universally accom- 

 plished by the association of individuals of two kinds, males 

 and females, living commonly in pairs or flocks, and each of 

 them characterized by peculiarities of structure and external 

 appearance. As this distinction prevails throughout the animal 

 kingdom, it is always necessary for obtaining a correct and 

 complete idea of a species, to bear in mind the peculiarities of 

 both sexes. Every one is familiar with the differences between 

 the cock and the hen, the lion and the lioness. Less promi- 

 nent peculiarities are observed in most vertebrata. Among the 

 articulata, . the differences are no less striking, the males being 

 often of a different shape and colour, 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. Among the mollusca the females have often a wider 

 shell. 



432. Even higher distinctions than specific ones are based 

 upon peculiarities of sex ; 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 pecu- 



