8 



EMBRYOLOGY. 



the ordinary condition of the protoplasm and nucleus of most animal 



cells. 



The vitellus (tigs. 1 and 3 n.d) rarely appears homogeneous, mucila- 

 ginous, and translucent, like the protoplasm of most cells; it is 

 ordinarily opaque and coarsely granular. This results from the 

 fact that the egg-cell, during its development in the ovary, stores 

 up in itself nutritive materials, or reserve stuff's. These consist of 

 fat, of albuminous substances, and of mixtures of the two, and 

 are described, according to their form, as larger and smaller yolk- 

 spherules, yolk-plates, etc. Later, when the process of development 

 is in progress, they are gradually used up in the growth and for 

 the increase of the embryonic cells. The fundamental substance 



of the egg, in which the reserve stuffs 

 just now referred to are imbedded, is 

 protoplasm, physiologically the most in- 

 teresting and important of substances, 

 because in it take place, as we infer 

 from many phenomena, the essential 

 life-processes. 



We must therefore distinguish in 

 the yolk, in accordance with the sug- 

 gestion of VAN BENEDEN, (1) the egg- 

 protoplasm, and (2) the yolk-substance, 

 or deutoplcusm, which is of a chemi- 

 cally different nature, and is stored 

 up in the former. 

 When the deposition of reserve materials takes place to a great 

 degree, the really essential substance, the egg-protoplasm, may 

 become almost entirely obscured by it (figs. 3, 4). The protoplasm 

 then fills up the small interstices between the closely packed yolk- 

 globules, yolk-cakes, or lamellae, as mortar does those between the 

 stones in masonry, and appears in sections only as a delicate net- 

 work, in the smaller and larger meshes of which lie the yolk-elements. 

 Only at the surface of the egg is the egg-plasm constantly present 

 as a thicker or thinner continuous cortical layer. 



The germinative vesicle usually occupies the middle of the egg. 

 It is the largest nuclear structure in the animal body, and its 

 diameter generally increases with the size of the egg. 



The germinative vesicle (figs. 1, 2) is separated from the yolk by 

 a firm membrane, which may often be distinctly demonstrated, and 

 which surrounds various included components : nuclear liquid (Kern- 



\ 



Fig. 1. Immature egg from the ovary 

 of an Echinoderm. The large ger- 

 minative vesicle shows a germinative 

 dot, or nucleolus, in a network of 

 filaments, the nuclear network. 



