952 SPECIAL PHYSIOLOGY. 



central fluid part becomes clear. The segmentation of the entire yolk, 

 has been observed in all cases of development from holoblastic ova, 

 even in Non-vertebrate animals. In the eggs of certain Branchio- 

 gasteropods, a remarkable revolution of the yolk takes place, subse- 

 quently to the period of its segmentation, the yolk turning first in one 

 way and then in another, within the vitelline membrane ; this is said 

 to depend on ciliary movements. In the meroblastic ova of the Cepha- 

 lopods, certain Fishes, Reptiles, and Birds, only a part of the yolk, 

 viz., the germ-yolk, in the neighborhood of the germinal vesicle, under- 

 goes this segmentation, the result being the formation of the germinal 

 disc or cicatricula, already mentioned, from which, however, an exten- 

 sion of a single layer of cells proceeds over the unsegmented portion 

 of the yolk, forming the so-called germinal sac. 



This yolk-cleavage is a phenomenon of cell-division. It is stated by 

 Miiller, Gegenbauer, and Leydig, that the germinal vesicle, or nucleus 

 of the cell of the ovum, and the germinal spot, or nucleolus of that 

 cell, are directly concerned in this process. The nucleolus and nucleus 

 are said to divide successively into two, four, and many nuclei ; around 

 which the yolk, apparently governed by these successive subdivisions 

 of the nucleus, gathers for the development of the nucleated cells. In 

 the last generation, these cells acquire distinct cell-walls. According 

 to others, in certain animals, the germinal vesicle and germinal spot, 

 the primitive nucleus and nucleolus disappear completely, immediately 

 after fertilization occurs, and their future destiny cannot be traced ; 

 but & fresh cell or nucleus is supposed to be formed, around which the 

 yolk gathers, and by the repeated subdivision of this, into two, four, 

 eight, and so forth, the cleavage of the yolk is accomplished. The 

 first cell from which these arise has been named the embryo cell. Ac- 

 cording to the former view, the cells which appear on the surface of 

 the yolk, and out of a part of which, as, we shall immediately see, the 

 embryo itself begins to be evolved, are derived directly from a nucleus, 

 once forming a part of the maternal organism. According to the lat- 

 ter view, they, and therefore the embryo, are developed indirectly, or 

 independently, after the solution and diffusion of the primitive germinal 

 vesicle and spot, with the fertilizing agent, in the substance of the yolk. 

 In the flowering plants, also, the nucleus c-r germ-vesicle of the ovule, 

 or ovum-cell, is said, by some, not to participate -in the formation of 

 the cells from which the embryo plant is developed. Whichever of the 

 preceding views be adopted, the cleavage of the yolk is attributed, 

 however, to the formative force, known as the germ-force, resident in 

 the first-formed nucleus and nucleolus, and continued in all their suc- 

 cessive subdivisions. The germinal vesicle and spot, as already stated, 

 are said to disappear even in unfertilized Birds' ova. 



The formation of the layer of nucleated cells which constitute the 

 germ-sac of the holoblastic, or the germinal disc of the meroblastic, 

 ovum, as a result of the cleavage of the yolk, is altogether preliminary 

 to the formation of the embryo, which cannot yet be recognized. In 

 the Non-vertebrate holoblastic ovum, as, for example, in the Nematoid 

 Ascarides, these cells soon aggregate into a more or less coiled opaque 

 mass, which assumes the shape of the embryo, and then, by further 



