Bauhaus-Universität Weimar

Titel:
The Cyclopaedia of Anatomy and Physiology, vol. 5: Supplementary Volume
Person:
Todd, Robert Bentley
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit36060/148/
OVUM. 
[140] 
their appearance. According to most ovo- 
logists, the last globules formed by segmenta¬ 
tion are the nucleated organised cells im¬ 
mediately constituting the blastoderm a. A 
different view of the process, however, in 
Mammalia, has been taken by Bischoff, very 
decidedly set forth in his two most recent 
works on the development of the guinea-pig 
and the deer; according to which the last 
resulting spherules formed by segmentation 
are not true cells, and that previous to the 
formation of the blastodermic cells, the yolk- 
germ falls completely into an amorphous or 
homogeneous finely granular substance, out of 
which, secondarily, the blastodermic cells are 
produced by a process of cytogenesis. It 
seems probable that, in the different classes of 
animals, there may be considerable variety in 
the degree of perfection in organisation or ad¬ 
vance in cell-structure to which the segments 
of the yolk have attained at the period when 
the development of the embryo begins to ma¬ 
nifest itself. But in the higher animals at least 
the weight of evidence appears to me in favour 
of the view that the process of segmentation 
results directly in the formation of blastodermic 
] now established by the ob;er- 
vations of Reichert in Entozoa, in 1841, of 
Ransom in osseous fishes, and more particu¬ 
larly those of Remak in Batrachia, that a de¬ 
licate membrane is formed over the surface of 
each of the segments as they appear, and that 
the last and smallest segments possess a deli¬ 
cate membranous envelope, appear to show 
that, in these animals, each segment has the 
structure of an organised cell, and is very si¬ 
milar to, if not identical with, those of the 
blastodermic lamina. 
The origin of the embryo-cell is still in¬ 
volved in obscurity. Most ovologists are dis¬ 
posed to connect it in some way or other 
with the previously existing germinal vesicle, or 
some part of its contents, and more especially 
the nucleus. For the solution of this ques¬ 
tion, as already remarked, a more accurate 
knowledge of what happens to the germinal 
vesicle at the time of that disappearance which 
has been so commonly observed at the period 
of the maturation of the ova of almost all ani¬ 
mals, will be required. Does the macula re¬ 
main, as has been alleged by some, to form the 
nucleus or the whole of the embryo-cell? 
Or, in other cases, if the multiplied maculae 
are dispersed among the granules of the ger¬ 
minal yolk, are they again collected together 
into a mass or spherule to form the embryo¬ 
cell? Or, again is the embryo-cell formed 
out of other materials, and not neces-arily 
either partially connected with, or wholly de¬ 
rived from, the germinal vesicle ? And finally, 
might it not be, according to some recent ob¬ 
servations, such as those of J. Miiller on En- 
tochoncha and those of Remak on the frog, 
that the disappearance of the germinal vesicle 
is not attended with the dispersion of its con¬ 
tents, but is a phenomenon caused only in a 
certain number of animals by the solution of 
the delicate external w'all of the vesicle, and 
by some change in the position and consist¬ 
ence of its contents? Further observations 
will be required to determine this point ; but 
if in the meantime we regard it as most pro¬ 
bable that the embryo-cell is in some way or 
other connected in its origin with the germinal 
vesicle, we might further found upon this 
the speculative view that the blastodermic 
cells and the blastema from which unques¬ 
tionably, by a histogenetic process of cell-di¬ 
vision and multiplication, the various textures 
and organs of the animal body are produced, 
may be regarded as the descendants of the 
original cell-germ from which the ovum was 
developed combined with the sperm. We 
should thus trace the organic cellular connec¬ 
tion between the succession of parents and 
offspring, w bich I have stated to be one of the 
most general facts in organised nature. 
The observations respecting the very re¬ 
markable movements of the yolk, before and 
during the earlier stages of the segmenting 
process which have now been recorded by 
several physiologists, must excite the liveliest 
interest, and suggest subject for much reflection 
as to the evidence they may afford of the 
causes of this change, or, if w e may use the 
expression, of the forces by which segmenta¬ 
tion is brought about. There seems to be 
little doubt that the embryo-cell (and its nu¬ 
cleus first of all) is the earliest to become di¬ 
vided, and that the process of cleavage then 
proceeds from the surface of the segmenting 
mass inwards towards the cell ; but in what 
relation the nucleus, granular substance of 
the yolk, and oviceil-membrane stand to each 
other in this process, must be left to be de¬ 
termined by future researches. 
Of the other early changes in the ovum 
which immediately follow fecundation and 
precede embryonic development little need 
here be said. They consist principally in the 
greater degree of consolidation and compact¬ 
ness acquired by the germinal part of the 
yolk, and in the formation in most animals of 
a clear space between the surface of the yolk- 
substance and the enclosing vitelline mem¬ 
brane. It is in this clear space, or, as it has 
been called by Newport, respiratory chamber, 
that the spermatozoa have been observed in 
those instances in which they have been as¬ 
certained to penetrate into the cavity of the 
ovum. There is another phenomenon of the 
same period, which has now been so frequently 
observed, and which is of so peculiar a nature, 
that it must not be passed over without no¬ 
tice ; I allude to the appearance in the re¬ 
spiratory space of one or more clear and 
highly refracting spherules, nearly of the size 
of the germinal vesicle, but quite independent 
of it. These clear hyaline-like globules have 
been observed in the ova of Gasteropodous Mol¬ 
lusca after fecundation by almost all those who^ 
have attended to the ovology of this class of 
animals, among whom may be mentioned 
Dumortier, Pouchet, Van Beneden, Nord- 
mann, and Vogt; in the Annelida by Quatre- 
fages ; in Mammalia by Bischoff' and Barry ; 
and in Batrachia by Newport. From the 
observations of Quatrefages in Hermella they 
appear to be excluded or expressed, as it 
were, from the clear basement-substance of
        

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