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 Metozoa, 

 are all multicellular in the adult condition, and have, 

 except in some of the lowest groups, a more or less 

 elaborate structure 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 microscopic examination of the sub- 

 stance 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 existence, 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 vesi- 

 cle 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. 



