174 SURVEY OF CELL-LIFE. 
lages of the tadpole, or larger corpuscles may be subsequently 
formed in the interior of hollow nuclei, for instance, the 
innumerable corpuscles in the germinal vesicle of the fish, 
and fat-globules in the nucleus of the fat-cells m the cranial 
cavity of fishes. 
The nucleus, in most instances, contains one or two, more 
rarely three or four small dark corpuscles, the nucleoh. Their 
size varies from that of a spot which is scarcely discernible to 
that of Wagner’s spot (macula germinativa) in the germinal vesi- 
cle. Nucleoli cannot be distinctly recognized in all cell-nuclei. 
They may be distinguished from the larger corpuscles, which are 
sometimes developed in certain hollow nuclei, from the cireum- 
stance of their being formed at a much earlier period; they 
exist, indeed, before the cell-nucleus. They are placed eccen- 
trically in the round nuclei, and in the hollow ones are dis- 
tinctly seen to lie upon the internal surface of the wall. It is 
very difficult to ascertain their nature; it may also vary very 
much in different cells, They sometimes appear to be capable 
of considerable enlargement, as in the nuclei of the fat-cells in 
the cranial cavity of the fish, and in such instances often have 
the appearance of fat. According to Schleiden, hollow nucleoli 
also frequently occur in plants. 
Most cell-nuclei agree in the peculiarity of not being dis- 
solved, or rendered transparent by acetic acid, at least not 
rapidly so, whilst the cell-membrane of animal cells is in 
most cases very sensitive to its action. Some cells, (such 
as those of the yelk-cavity of the egg, plate II, fig. 3,) 
which have no perceptible nucleus of the ordinary form, ex- 
hibit a globule having the appearance of a fat-globule, which 
grows as the cell expands, though not in the same proportion, 
and was probably formed previous to the cell. Whether such 
a globule have the signification of a nucleus or not, must re- 
main an undecided question. 
The formation of the cell-nucleus. In plants, according to 
Schleiden, the nucleolus is first formed, and the nucleus around 
it. The same appears to be the case in animals. According 
to the observations of R. Wagner on the development of ova 
in the ovary of Agrion virgo,' the germinal spot is first 
' See Wagner, Beitraige zur Geschichte der Zeugung und Entwickelung; Erster 
Beitrag., tab. II, fig. 1. 
