MEROBLASTIC OVA. 941 



a part or product of a pre-existing parent. No satisfactory proof has 

 yet been adduced, of the spontaneous origin of such a cell. ' Hence, 

 the doctrine of spontaneous generation collapses from failure of proof. 



The Ovum considered generally. 



The parts seen in an unfertilized animal ovum, as already stated, 

 are the cell-wall, the contents, the nucleus, and the contained nucleo- 

 lus, Fig. 116. The delicate cell-wall constitutes the vitelline membrane, 

 or yolk-sac. The more or less transparent, granular, or colored con- 

 tents, constitute the yolk. The nucleus is a transparent, solid, or 

 vesicular body, here named the germinal vesicle, or vesicle of Purkinje. 

 Lastly, the nucleolus within it, is a fine granular or vesicular corpus- 

 cle, called the germinal spot. The germinal vesicle and spot are the 

 essential parts, or active centres of growth of the ovum. 



As to the Vegetable Kingdom, in the higher plants, which are produced 

 from seeds, a part exists in the seed, known as the ovule; within this, which 

 is a vegetable cell, is found the germ-vesicle, a structure homologous with the 

 germinal vesicle of the animal ovum. Like it, its future development requires 

 the co-operation of a fertilizing agent, which is here derived from the pollen- 

 cells. In the lower or flowerless plants, the spores are usually fertilized by 

 movable filaments named zoosperms, or by simpler elements. 



The size of the ovum of different animals differs very much, not in 

 accordance with the size of the parent animal, but rather with the 

 course and conditions of development of the future embryo. The 

 difference in size depends almost entirely upon the quantity of the 

 yolk or cell-contents. The character of this yolk also varies : some- 

 times it is so finely granular and colorless as to appear clear; whilst, 

 in other cases, it is so distinctly granular and colored, as to contain 

 large granules and even vesicles, with oil-globules, to be more or less 

 opaque, and to present a pale or deep yellowish hue. 



The yolk is a most important constituent of the ovum. In all cases 

 it informative, yielding material for the first formation of the embryo; 

 sometimes it is also nutritive, or provides nourishment for it during a 

 considerable period of its growth. 



In one series of animals, oviparous, the development of the embryo 

 within the ovum occurs entirely after the latter has been deposited by 

 the parent animal ; whereas, in another series, often viviparous, the 

 embryo is more or less developed within the parent. In the first case 

 nutrient material must be specially provided in the ovum for the future 

 embryo, the various organs of which are developed at the expense of 

 the yolk-contents, until the young animal has reached a phase of de- 

 velopment in which it can take external materials for its future nour- 

 ishment. In such cases the yolk is comparatively large in quantity, 

 and rich in organic granular contents, opaque, and colored ; it is 

 chiefly nutritive, and, in a small part only, formative. Such ova are 

 named meroblastic (/^<><r, a part, /?Aa<rro, a germ) ; they include the 

 eggs of the higher Crustacea and Arachnida, those of the Cephalopods, 

 and those of the Osseous and Plagiostomatous Fishes, of Reptiles and 



