chap, xiv.] DEVELOPMENT OF METAZOA. 525 



to forage for themselves, provide them with food. All 

 Mammals suckle their young, and the ascent in the 

 series appropriately finds its termination in man, who 

 alone has the idea of the family. 



CHAPTER XIY. 



THE DEVELOPMENT OF THE METAZOA. 



THE organs, described in the preceding chapter, 

 whether essential, as the testes or ovaries, or accessory, 

 as the ducts, glands, and intromittent organs, have in 

 common the function of providing for the union of the 

 male cell or spermatozoon and the female cell or ovum. 

 In all Metazoa, save those which multiply by budding 

 or by the parthenogenetic development of the egg, 

 this union of a male and female cell is the first step 

 in the history of a new individual ; it may be effected 

 either within the body of the female, as in Birds and 

 Mammals, or it may be effected externally to it as in 

 the frog, the perch, or the starfish. The ovum having 

 freed itself of the polar globules, and received within 

 itself the male or fertilising element, proceeds to 

 undergo division or segmentation this may regularly 

 affect all the parts of the egg, and the several seg- 

 ments may be all of the same size, or, as in the egg 

 of the hen, where there is an abundant supply of 

 yolk, division may go on actively in a small portion 

 only of the whole egg. It is particularly to be 

 borne in mind that the absence or presence of yolk 

 in small or larger quantities profoundly affects the 

 character of the segmentation. 



Regular segmentation, such as is seen in Am- 

 phioxus, obtains only in ova in which there is none 

 or but little yolk (aleoitlia I ova ; Lankester) ; it is 

 most common among the lowest Metazoa, such as the 



