100 A Sketch of Comparative Embryology. [February, 
3. The nucleus becomes larger, spherical, and assumes an 
eccentric position within the cell, while the meshes of the 
nuclear network are coarse, and few, and for the most part 
radiate from the nucleolus, which is large, distinct, highly 
refringent and placed eccentrically within the nucleus. 
4. The cellular network becomes very distinct, its interspaces are 
filled with ovoid or round, solid enclosures, which are 
usually if not always mainly of an albuminoid character. 
These enclosures form the part which is called the deuto- 
plasm by Edouard von Beneden and others. The deuto- 
plasm causes the egg-cells to be called the yolk, because it 
is the nutritive matter from which the protoplasm of the 
cell grows. The term yolk has no very exact scientific 
meaning, for it is used to designate sometimes the deuto- 
plasm alone, sometimes the whole egg, as when the seg- 
mentation of the yolk is spoken of. 
5. A cell membrane appears, and usually acquires considerable 
thickness, 
A typical mature egg-cell is shown in Fig. 1, which represent 
the ripe ovum of Zozopneustes lividus, the common sea-urchin of 
Europe. The nucleus is proportionately larger than in the eggs 
of many other animals, its contents are fluid except the net-work 
and the nocens (z), which latter frequently has one or more 
mu vacuoles. In some cases there are several 
or many nucleoli, as in osseous fishes, but 
the meaning of this difference is absolutely 
| unknown. Moreover, this egg is unlike 
that of many animals in that the yolk 
spherules or the deutoplasm granules are 
comparatively small, while in some ani- 
TATU mals, especially those with larger eggs, the 
sea- a aa A granules are larger. If these variations 
wig. are borne in mind the figure given may be 
accepted as a correct representation of a mature egg-cell. 
I am inclined to think that besides these peculiarities the ripe 
egg-cell shows a distinction between a thin denser peripheral 
layer of protoplasm immediately under the vitelline membrane 
and a central portion, which alone contains deutoplasm, recalling 
the differentiation of the ectosarc and endosarc-in the Amæba. 
This feature has been observed in several cases, and further 
research may demonstrate it to be common to all eggs. 
