SURVEY OF CELL-LIFE. 175 
formed, and around that the germinal vesicle, which is the 
nucleus of the ovum-cell, Eizelle.'| The youngest germinal 
vesicle there represented by Wagner, appears to be hollow. 
This is not generally the case, however, in the formation of 
cell-nuclei. Plate III, fig. 1, e, appears to be a cell-nucleus 
of a cartilage-cell in the act of forming. A small round 
corpuscle is there seen, surrounded by some minutely gra- 
nulous substance, whilst the rest of the cytoblastema is 
homogeneous. This granulous substance is gradually lost 
around the object; at a subsequent period it begins to 
be sharply defined, and then exhibits the form of a cell- 
nucleus, which continues to grow for a certain period. (See 
pl. III, fig. 1, a, 6.) Such a nucleus usually appears solid 
in the first imstance, and many nuclei remain in this con- 
dition; in others, on the contrary, the portion of the sub- 
stance situated nearest to the external surface continually 
becomes darker, and not unfrequently at last forms a dis- 
tinctly perceptible membrane, so that the nucleus is hollow 
in such instances. The formative process of the nucleus 
may, accordingly, be conceived to be as follows: A nucle- 
olus is first formed; around this a stratum of substance 
is deposited, which is usually minutely granulous, but not as 
yet sharply defined on the outside. As new molecules are 
constantly being deposited in this stratum between those 
already present, and as this takes place within a precise dis- 
tance of the nucleolus only, the stratum becomes defined 
externally, and a cell-nucleus having a more or less sharp con- 
tour is formed. The nucleus grows by a continuous depo- 
sition of new molecules between those already existing, that 
is, by intussusception. If this go on equably throughout the 
entire thickness of the stratum, the nucleus may remain solid ; 
but if it go on more vigorously in the external part, the latter 
will become more dense, and may become hardened into 
a membrane, and such are the hollow nuclei. The circum- 
stance of the layer generally becoming more dense on its 
exterior, may be explained by the fact that the nutritive fluid 
is conveyed to it from the outside, and is therefore more con- 
centrated in that situation. Now if the deposition of the new 
' See the Supplement. 
