257 
nal vesicle, as Metschnikow has demonstrated.! The much 
more recent observations of Kupffer on the development of 
the Ascidians have brought to light a most remarkable fact ; 
it is the development in an endogenous manner of an entire 
layer of cells under the membrane of the ovarian egg, and 
that, too, before fecundation. These cells are formed at the 
expense of a continuous layer of finely granular protoplasm, 
and the nuclei are said to appear in the cells after their 
individualisation.? 
I have elsewhere expressed my views as to the develop- 
ment of the nucleus in the fecundated egg in the place of 
the germinal vesicle.* 
In the vegetable kingdom also, certain examples of this 
mode of cell-multiplication are known, to which botanists 
give the name of “freie Zellenbildung.”* Such are the 
formation of the embryonic vesicle, and of the first cells of 
the endosperm. 
Seeing that the cases are few in which endogenous cell- 
formation is demonstrated, my observations on the develop- 
ment of the Gregarine have interest from this point of view, 
since here certainly the nucleolus and nucleus are developed 
in the cytod by endogenous formation. When I observed, 
for the first time, the disappearance and reappearance of the 
nucleolus in the nucleus of the Gregarina, it seemed to me 
that these facts tended to diminish the importance which one 
is accustomed to attribute to the nucleolus as a constituent 
part of the cell. It is, therefore, with astonishment that I 
saw the nucleolus appear before the nucleus in the progres- 
sive development of the cell; and as a result, one must admit 
a stage intermediate between the cytod and the nucleus- 
bearing cell; this stage being that of the cytod provided 
with a nucleolus. 
This fact of the appearance of the nucleolus before the 
nuclear layer confirms the view of the illustrious founder of 
animal histology, who held that the nucleolus appears first, 
then the nuclear layer, and finally the body of the cell. 
The existence of the Monera, which have been the origin 
of all living beings, and whose extreme simplicity is found 
again in the youngest Gregarine, proves the existence of 
‘ Metschnikow, “Embryologische Studien an Insecten,” ‘Zeitschrift 
fiir Wiss. Zool.,’ Bd. xvi. 
> Kupffer, Joc. cit. This cellular layer persists during the entire embry- 
onic development of the Ascidian, and is destined to become the test or 
external layer of the mantle.—Kupffer. 
3 Edouard Van Beneden, ‘ Recherches sur la Composition et la Significa- 
tion de lceuf;’ ‘Mem. de l’Acad. Royale des Sci. de Belg,’ t. xxxiv. 
4 Sachs, ‘ Lehrbuch der Botanik,’ p. 11. 
