DR. MARTIN BARRY’S RESEARCHES IN EMBRYOLOGY. 
549 
Schleiden, I refer to what has been above stated regarding the changes in appear- 
ance presented by the nucleus, from the resolution of its outer portion into cells, and 
the continual origin in the interior of the foundations of new cells, and hence the vary- 
ing size of the central pellucid cavity (par. 327.) ; believing that the apparent growth 
and diminution, and the final “ absorption of the nucleus,” supposed by Schleiden, as 
well as the nature of his nucleolus, are to be thus explained. If so, however, it will 
be seen that the formation of the nucleus precedes that of the nucleolus, instead of 
being consequent upon it, as supposed by Schleiden -f*. There seems, in fact, to arise 
at the surface of the nucleus a succession of objects, each of which, in turn, is a nu- 
cleolus : and this after the formation of the cell. More particularly considered, the 
process seems to be as follows. The progressive resolution of the germinal spot into 
layers of incipient cells, has shown us that the formation of each successive layer is 
preceded by a finely granular appearance, the latter surrounding a pellucid fluid j;. 
It seems to be in this finely granular part that the discs or incipient cells come into 
view (Plate XXIV. fig. 192.). On the enlargement of the latter, an inner part, pre- 
viously pellucid, is found to have become finely granular; but this also surrounds a 
pellucid central fluid, and so on. It would thus seem that the central pellucid fluid 
becomes finely granular. The ever- varying appearance called the nucleolus, I appre- 
hend to arise from the refraction of light; the degree of which, and the particular situ- 
ation in which it occurs, depend chiefly on two circumstances, namely, the condition of 
the granular substance by which the pellucid fluid is immediately surrounded, and the 
quantity of this fluid which is present. Schleiden considered the presence of two 
(or more) nucleoli as exceptions. I am disposed to regard them as constant at a cer- 
tain period ; and as denoting the origin of the two (or more) essential cells. 
386. The discoveries of my friend Professor Schwann § — on the correspondence in 
the elementary structure of Animals and Plants — were referred to on a former occa- 
sion ||, as being among the most important that had ever resulted from microscopic 
observation^). That excellent observer, however, did not attempt to conceal the 
fact, that he had met with certain difficulties, and was obliged to leave them for future 
explanation. For instance, with reference to the objects he considered hollow and 
pellucid nuclei, — differing as he found these to do in their appearance from the ordi- 
nary “ cytoblast,” — he remarks •f~f v : “Observation on the transition-steps from the 
characteristic form of the nuclei of cells, must furnish the required explanation of 
t This view of Schleiden on the mode of origin of the nucleus of the vegetable cell has been adopted by 
authors on the cells of animal tissues. Valentin, having given an abstract of the work of Schwann on Cells, 
remarks, “ Nach diesen, so wie nach den Erfahrungen von J. Muller, Henle und dem Ref. stellt es sicli 
also als Grundschema der ersten Bildung der Gewebtheile in der Thierwelt heraus, class in einer Fliissigkeit, 
cytoblastema, sich Kdrnchen, Nucleoli niederschlagen und dass um diese sich grossere feinere Korper, Nuclei 
bilden.” — Repertorium, 1839, II. p. 284. 
X See also the rudimental embryo, “ Second Series,” l. c., Plate VII. fig. 121 B, 121 C. 
§ Mikroskopisclie Untersuchungen, &c. || “ Second Series,” /. c., par. 290. 
Those discoveries have also been mentioned in the present memoir (par. 324.). ft L. c., p. 211. 
