Bauhaus-Universität Weimar

842 
DEVELOPMENT OF THE OVUM OF MAMMALIA. 
gives origin to two others, making four: each of these four in turn gives origin to two, 
by which the number is increased to eight; and this mode of augmentation continues 
until the germ consists of a mulberry-like object, the cells of which are so numerons 
as not to admit of being counted. Together with a doubling of the number of the 
cells, there occurs a diminution of their size. Every cell, whatever its minuteness, is 
found filled with the foundations of new cells into which its nucleus has been resolved. 
These foundations of new cells are arranged in concentric layers around a pellucid 
point. Each cell in fact exhibits the same process of cellular development as the 
original parent cell—the germinal vesicle. The foregoing changes usually take place 
in the ovum during its passage through the Fallopian tube. When it has entered the 
uterus, a layer of cells of the same kind as those forming the mulberry-like body makes 
its appearance on the whole of the inner surface of the membrane which invests the 
yolk. The mulberry-like structure then passes from the centre of the yolk to a certain 
part of that layer; the vesicles of the latter coalesce with those of the former, where 
the two sets are in contact, to form a membrane—the future amnion; and the interior 
of the mulberry-like structure is now seen to be occupied by a large vesicle contain¬ 
ing a fluid and dark granules. In the centre of the fluid of this vesicle is a spherical 
body, composed of a substance having a finely granular appearance, and containing a 
cavity filled with a colourless and pellucid fluid. This hollow spherical body seems 
to be the true germ. The vesicle containing it disappears, and in its place is seen an 
elliptical depression filled with a pellucid fluid. In the centre of this depression 
(which appears to correspond to the area pellucida of the bird’s egg) is the germ still 
presenting the appearance of a hollow sphere. 
From the germ the embryo now begins to be formed. The germ separates into a 
central and a peripheral portion, both of which, at first appearing granular, are subse¬ 
quently found to consist of vesicles. The central portion occupies the situation of the 
future brain, and soon presents a pointed process. This process becomes a hollow 
tube, exhibiting an enlargement at its caudal extremity, which indicates the situation 
of the future sinus rhomboidalis. Up to a certain period new layers of vesicles or 
cells come into view in the interior of the central portion of the germ, parts previously 
seen being pushed further out. 
According to Dr. Barry, there is no structure in the mammiferous 
ovum entitled to be denominated the “germinal membrane.” The 
“amnion” is formed, as has already been mentioned, from an epithe¬ 
lium-like layer of cells which lines the investing membrane of the ovum, 
and from the outer cells of the mulberry-like body, which together con¬ 
stitute a layer corresponding to the “lamina serosa” of authors. The 
“vascular lamina” of the umbilical vesicle arises as a hollow process 
originating from the germ and extending beneath the amnion so as to 
include the yolk.* 
The “yolk-sac” of the mammiferous ovum communicates with the 
intestine of the embryo, at first by a wide opening, and afterwards by 
a duct or hollow pedicle, the “ ductus omphalo-mesentericus,” which is 
accompanied by the same vessels as in the bird, the “vasa omphalo- 
mesenterica.” This yolk-sac of Mammalia is commonly called the 
“umbilical vesicle, or vesicula umbilicalis.” According to Von Baer’s 
researches, it has an external vascular layer, and an internal mucous 
layer, from which villous prolongations project into the yolk. These 
villi or folds, which are similar to those found in the yolk-sacs of birds, 
exist likewise in the umbilical vesicle of the human ovum. The “am¬ 
nion” was observed by Yon Baer to hold the same relation to the 
abdominal plates of the embryo, as in the bird; and it is doubtless 
developed in the same manner. The “ allantois” also, is developed by 
* This notice of Dr. Barry’s observations is introduced by the translator.
        

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