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/lit36060/68/
60 
OVUM. 
comparatively large veins which constitute 
the hollow spaces. The ends of these veins, 
then, look towards the inner surface of the 
membrane ; and the appearance of a divided 
cavity in some of the supposed follicles is 
merely caused by two or more veins meet¬ 
ing in a common dilatation at this place. 
The capillary vessels, in passing into these 
large commencements of the veins seem to 
converge from its circumference to its centre. 
In the enlarged ovarian capsules of the 
turtle, a somewhat similar arrangement may 
be observed ; but I have not had an oppor¬ 
tunity of tracing its relation to the blood¬ 
vessels ; nor have I had the means of ascer¬ 
taining whether anything of the same kind 
exists in other reptiles with large yolks. In 
the skate 1 have not been able to perceive 
any similar arrangement ; and in the Graafian 
vesicle of mammalia the lining membrane 
presents internally a smooth surface destitute 
of any appearance of depressions or of pecu¬ 
liar venous sinuses. 
The appearance which I have just now 
described had not escaped the notice of Von 
Baer ; for at p. 23 of his work on develop¬ 
ment, he mentions the existence of clearer 
points in the inner membrane of the theca, 
and states his opinion that they may be open 
mouths of blood-vessels, by means of which 
the yolk may be nourished by the direct 
access of blood to it. 
In the naked amphibia and osseous fishes, 
the ovaries (of which the general form has 
been previously noticed) present a still greater 
decrease in the proportion of the stroma to 
the ovicapsules and ova. These capsules are 
themselves also of much more delicate struc¬ 
ture than in the higher animals ; but the rela¬ 
tion of the ovules to the ovicapsules in their 
formation, and the mode of their escape by the 
rupture of the theca, are essentially analogous 
to those of birds and reptiles. In the earliest 
condition, it is true, the ovary may present 
a greater amount of solidity in some of these 
animals ; but from the prodigious number of 
the germs of the ovules and the small quan¬ 
tity of the ovarian stroma, as soon as the 
ovary has made some progress in develop¬ 
ment, it acquires the appearance rather of 
a mere mass of ova connected together by 
a membrane and fine thread-like pedicles, 
than of a solid or consistent organ containing 
them. The delicate ovicapsules containing 
the ovules embrace them closely as in the 
large-yolked group of animals, there being 
little or no fluid between the capsules and 
the vitelline membrane. 
The structure of the ovaries in the inver¬ 
tebrate animals presents so many varieties 
that it would occupy too much space to 
allude to them here. I refer the reader for 
information regarding them to the article 
Organs of Generation, and others on par¬ 
ticular classes and orders of animals in dif¬ 
ferent parts of this work. For our present 
purpose the structure of these organs has been 
sufficiently indicated in the previous section. 
In conclusion, it may be right to recapitulate 
the general nature of the ovary or formative 
organ in its relation to the production of ova. 
A comparison of the forms previously indicated 
leads to the general view that the ovary is to 
be regarded as analogous to the glandular 
organs. _ In the great majority of animals 
highest in the scale, the ovisacs are close fol¬ 
licles from which the product of formation 
(or secretion) escapes by the bursting of the 
wall of the follicle — in the highest animals, on 
the external surface of the organ, in those 
coming next in the series, towards an internal 
cavity. In other instances, principally among 
the lower animals, the structure is more ana¬ 
logous to that we are accustomed to consider 
as characteristic of the true glands, in which 
the secreted cellular product is formed within 
the same or a continuation of the tubular 
ducts themselves by which they make their 
escape. The more complex structure of the 
capsules in which the large-yolked ovules are 
produced in birds constitutes a special appa¬ 
ratus, which, though without follicular com¬ 
plication, may be looked upon as a modifica¬ 
tion or higher degree of development of the 
glandular structure of the ovary, provided 
for the rapid formation of the larger mass of 
nutritive substance which is present in these 
ova. 
§ 4. More detailed description of the ovum 
of birds as the type of the ls£ group. 
Having in the previous section given a 
sketch of the general resemblances and dif¬ 
ferences observed among the ova of various 
animals, I now proceed to describe more in 
detail an example from each of the three 
groups previously distinguished, and more 
particularly those of Birds and Mammalia, 
which demand the greatest share of our atten¬ 
tion in the study of development ; and first 
as to the ovum of the common fowl. 
Quantity of matter, composition, fyc. — The 
average dimensions of the fowl’s egg in this 
country are the following : The long diameter 
2f inches, short diameter If inch. The aver¬ 
age weight of eggs of this size is a little more 
than 2 oz. avoird., or 920 grains.* 
The extremes in weight which I have ob¬ 
served among eggs of the fowl naturally formed 
are 750 and 1060 grains. Double-yolked 
eggs are, as might be expected, much larger, 
reaching often a weight of 1400 grains, or 
3f oz. 
The yolk weighs about a third of the 
whole ; the albumen, membrane, and shell 
forming the remaining two thirds. These 
parts of the egg are in the following propor¬ 
tions to each other in 100 parts; the albu- 
* The following is a comparative view of the 
average size and weight of the eggs of the com¬ 
mon fowl, duck, turkey, and goose. 
Length 
Breadth 
Weight 
(in inches). 
(in grains). 
Fowl - 
2-25 
1-7 
920 nearly 2 oz. 
Duck - 
2-5 
1-75 
1100 
2£ oz. 
Turkey 
2-7 
1-9 
1300 
3 oz. 
Goose - 
3'3 
2-4 
2600 
6 oz.