282 Scientific Intelligence. 
in support of this hypothesis, we find that they may be reduced, as M. 
Vogt has remarked, to a single observation directly made on cartilage; 
and yet this observation, presented by Schwann himself as very doubt- 
ful, has been shown to be false by the researches of M. Vogt on the 
cartilage of the accoucheur-toad. In fact, in a very large number of 
cases, the nucleolus to which the theory attributes the exclusive privi- 
lege of causing the amorphous matter to produce the cell-walls—in a 
very large number of cases, I say, the nucleolus never appears free 
and isolated in the midst of the cytoblastema. On the contrary, we 
always find that this corpuscle, even from the first commencement of 
its appearance; is still enclosed in the cavity of the cell, which is pre-_ 
viously formed, frequent instances of which we find in the tissues of 
the embryo of osseous fishes; it is evident that in these cases at least 
the nucleolus has taken no part in the formation of the cell, as it was 
not in existence when the latter was produced. In other cases, this 
corpuscle did not appear at any period of the existence of the cells, 
and consequently we should have no motive for making it intervene as 
a determining cause, since it does not leave to the theory even the pre- 
text of co-existence. This may be easily verified by studying the de- 
velopment of the large cells which form the internal expansion of the 
umbilical vesicle of serpents. 
Hence the tardy appearance of the nucleolus in certain cases, and 
its total absence in others, form a serious impediment to the theory 
which locates the exclusive determining cause of all cell-formation in 
the pre-existence of this corpuscle. This also shakes the very founda- 
tions of the doctrine, and tends at the least to restrain its application. 
As regards the cytoblast or nucleus, M. Vogt has already shown that 
it has no influence on the formation of the cell-walls of the embryo of the 
osseous fishes; I have been enabled to convince myself that the large 
diaphanous vesicles in the spinal cord of the Batrachia do not appear 
until afier the production of the parietal membrane of these vesicles. 
But because the intervention of the nucleolus is not always necessary 
for the formation of the cells, and because the cytoblast or the nucleus 
does not itself, in a certain number of cases, retain the function assign- 
ed to it by theory, must we necessarily conclude that the cells are never 
developed around a centre upon which the forming walls would mould 
themselves? Undoubtedly we shall have frequent opportunities of ob- 
serving limited masses of matter becoming coated with an envelope; 
and thus constituting the contents of the pouch which is formed at their 
periphery ; but we shall then remark, that in most of these cases this 
happens in a very different manner from what the theory supposes ; for 
the matter which has constituted the centre, instead of being absorbed 
by the parietal membrane, to make room for the cellular contents sub- 
