SECTION II. — THE METAZOA 



While the Protozoa are predominantly unicellular, and of 

 extremely simple structure, the rest of the animal kingdom, 

 grouped together under the comprehensive title of Metazoa, 

 are all multicellular in the adult condition, and have, except 

 in some of the lowest groups, a more or less elaborate struc- 

 ture owing to the presence of complicated systems of organs 

 for carrying on the various functions of animal life. Such 

 an animal as a lobster or a frog, for example, may readily 

 be ascertained to be made up of a complicated system of 

 parts, — skeleton, muscles, digestive organs, blood vessels, 

 and so on, — and it requires only the most superficial micro- 

 scopic examination of the substance of these various parts 

 to render it evident that each is built up of an immense 

 multitude of cells. A lobster or a frog, however, or any 

 other Metazoan, consists, in the earliest stage of its exist- 

 ence, of a single cell, the oosperm, formed by the union 

 of a male cell or sperm with a female cell or ovum. The 

 ovum (Fig. 25) is usually spherical in shape, with one or 

 more enclosing membranes, with cell-protoplasm enclosing 

 a large nucleus (germinal vesicle, as it is often termed in 

 this case), in which are contained one or more small, 

 rounded bodies {germinal spot or spots). The ovum may 

 contain, in addition to the protoplasm, a quantity of non- 

 protoplasmic material or yolk. 



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