Bauhaus-Universität Weimar

Microscopical researches into the accordance in the structure and growth of animals and plants
Schwann, Theodor Schleyden
brane, that is, from such cells as are met with in the area 
pellucida, so that the embryo is composed, partly of small 
cells without nuclei, and partly of cells furnished with the cha¬ 
racteristic nucleus. It presents, however, besides them, an 
extraordinary quantity of simple cell-nuclei with nucleoli, around 
which no cells have as yet formed. 
I have made but few researches with respect to the structure 
of the vascular layer, and from them, I could not (with the 
exception of the vessels themselves and the blood) detect any 
such essential difference between it and the mucous layer, as 
was exhibited between the latter and the serous layer. As, how¬ 
ever, the formation of the vessels themselves, although it ap¬ 
pears to depend upon a production of cells, is not a process pe¬ 
culiar to the germinal membrane, we shall defer it, to be re¬ 
sumed at a subsequent stage of our investigation. 
I have not ascertained the relation which these cells of the 
layers of the germinal membrane have to the primitive globules 
of the membrane before incubation, or within eight hours after 
that process has commenced ; but inasmuch as it is probable 
that at least one of those kinds of cells owes its origin to the 
development of the primitive globules, we may be permitted to 
suppose that those globules are likewise cells. 
For the purpose of giving, in outline, a connected view of the 
changes which the egg undergoes, from its first formation up 
to the period at which the actual development of the embryo 
commences,—in so far as the foregoing, more or less complete, 
observations enable us to form a provisional conception of the 
process of development,—we will proceed on the understanding, 
that the germ-vesicle is the nucleus of the yelk-cell ; at the same 
time, however, we expressly refer the reader to the more de¬ 
tailed statement above furnished for the certainty both of this 
and of every other separate point which occurs in the following 
exposition. It is probable that the germ-vesicle is the first struc¬ 
ture, and that the yelk-cell forms around it as its cell-nucleus. 
Both advance in growth, the latter, however, much more rapidly 
than the former. A precipitate, the commencement of the ger¬ 
minal membrane, next forms around the germ-vesicle. Young 
cells are simultaneously formed in the remaining space of the 
yelk-cell, these are the cells of the subsequent yelk-cavity. Then 
cells of another kind originate beneath the vitelline membrane,


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