Bauhaus-Universität Weimar

The Cyclopaedia of Anatomy and Physiology, vol. 5: Supplementary Volume
Todd, Robert Bentley
of late attracted considerable attention, and 
appears to have been first clearly stated by 
Reichert in 1840, and afterwards in 1843 * ; 
and in accordance with the views of that au¬ 
thor, we may with propriety distinguish the 
formative (or germinal) from the nutritive 
parts of the yolk. In the fowl’s egg, for ex¬ 
ample (in which it must be admitted these 
two parts were long confounded together), 
the cicatricula, together with its so called 
nucleus, and a part, perhaps, of the lighter- 
coloured substance which occupies the centre 
of the yolk and the canal extending from 
it to the cicatricula, constitutes the formative 
or germinal part ; and the larger mass of the 
more deeply-coloured portion of the yolk 
forms the nutritive vitelline substance. In 
the mammiferous ovum, on the other hand, 
the latter part is either entirely absent, or is 
in small quantity, and the whole of the yolk 
substance may be looked upon as directly 
formative, or as analogous to that which forms 
only the cicatricula of the fowl’s egg. Among 
more recent writers the distinction of these 
parts has been particularly insisted upon, and 
illustrated by M. Costef, and also by Messrs. 
Prévost and Lebert.J 
The difference in the relative amount of 
■the formative and nutritive yolk substance, as 
well as in the size of the whole ovum, in birds 
and mammalia, is manifestly to be regarded as 
more immediately connected with the dif¬ 
ferent manner in which the embryo is to be 
supplied with the materials necessary for its 
growth in the two cases ; in the oviparous 
mode of development, the whole amount of 
nourishment required being provided in the 
egg itself, and detached along with it from the 
parent ; in the truly viviparous mode, a con¬ 
tinual addition of new materials for growth, 
being made by transmission from the maternal 
parent in the placenta, or in some analogous 
structure, which accompanies utero-gestation. 
The smaller proportional size of the nutri¬ 
tive part in Batrachia and Osseous Fishes 
(though most of these animals are truly ovi¬ 
parous), may be attributed to the very early 
period of development, and consequent small 
size of the embryo at the time when in these 
aquatic animals it leaves the egg, and, taking 
upon itself an independent life, gathers nou¬ 
rishment in the same manner as the adult 
animal. $ 
* Entwickelungsleben im Wirbelthierreich ; 4to. 
Berlin, 1840 ; and in Beiträge zur Kenntniss des 
heutigen Entwickelungsgeschichte; 8vo. Berlin, 
1843, p. 22. _ . 
t Cours d’Embryogenie Comparée, tom. i. Fans, 
1837 ; and Histoire gen. et partie, du Développement 
de l’Homme et des Animaux, Paris, torn. i. 4to., 
$ In Annal, des Seien. Hat. for 1844. 3rd Ser. 
tom. i. p. 193 and 265. , 
K At the same time it is to be kept m mind that 
there are exceptions to these relations, which make 
it extremely difficult to state any general law of 
connection between the structure of the ovum and 
the mode of gestation and place of development of 
the embryo ; as in the case of a few of the lizards 
and serpents, and some cartilaginous fishes, in which 
although the egg agrees in general structure with 
The above arrangement is by no means 
offered as exhausting the divisions which 
might be formed of the ova of animals, but 
rather as bringing forward prominently the 
most remarkable characteristics of those of 
vertebrata. It is not improbable that a more 
accurate acquaintance with the structure of the 
ova in the animals thus grouped, and more 
especially of the Invertebrata, may lead to some 
considerable modifications of the divisions here 
adopted; but the main distinction upon which 
they are founded is so important, that even 
with our present incomplete acquaintance with 
them, it seems advisable to call attention to 
it at this place. As I shall have occasion 
to refer frequently to these groups in the sub¬ 
sequent description of the ova of various 
animals, in the absence of more appropriate 
appellations, I will, for the sake of brevity, 
designate them severally, as follows,—viz,, 1st 
group, Small-yolked ova, as in Mammalia ; 2nd 
group, Large-yolked ova, as in Birds, Scaly 
Reptiles, and Cartilaginous Fishes ; 3rd group, 
Middle-sized yolked ova, as in Batrachia, Os¬ 
seous Fishes, &c. 
§ 2. Further comparison of the ova of animals 
in general, as respects their size, number, form, 
and the relation of their parts. 
Size of ova. — In addition to what has been 
said on this subject in the previous section, it 
may farther be remarked that, in the second 
and third groups, the size of the ova of dif¬ 
ferent genera and species is to a certain extent 
proportional to that of the adult animal, or of 
the fully-developed foetus; but in the first 
group, or at least in Mammalia, in which the 
nutritive part of the yolk may be considered 
as wholly or nearly entirely absent, there is a 
much greater uniformity in the size of the 
ova; and, accordingly, the largest mammi¬ 
ferous animal may take origin from an ovum 
which, when mature, is even smaller than 
those of species of animals many hundred 
times less in bulk ; while in the class of 
Birds we observe the nearly regular increase 
of the size of the ovum in proportion to that 
of the parent animal, from the smallest hum¬ 
ming bird up to the ostrich, or the still larger 
egg of the Æpyornis, an extinct bird, of which 
some of the bones, along with the eggs, have 
recently been discovered in Madagascar.* 
that of animals which are generally oviparous, it is 
retained in the oviduct or uterus of the female during 
a part or even the whole of the time of foetal develop¬ 
ment ; and there are also exceptions in the third 
group—viz. that of batrachia and osseous fishes as 
in the Land Salamander and Viviparous Blenny. 
To this mode of gestation the name of Ovoviviparous 
has been given. There are many varieties of a 
similar kind among the Invertebrata, and on the 
whole it may he stated that there is no constant 
correspondence between the size of the ovum and 
the mode of gestation. The Marsupiata also, and 
the Monotremata among the Mammalia, exhibit 
interesting modifications, in the first a partial, and 
in the second, probably a complete residence of the 
ovum in the uterus of the female parent during de¬ 
velopment ; while the ovum in these animals ap¬ 
proaches, in some respects, the type which is more 
commonly oviparous. 
* The circumference of this extraordinary egg


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