THE OVUM IN THE AMPHIBIA. 
183 
duct. During the remainder of its passage the egg gains two other distinct layers of 
similar investment, which, together, we afterwards recognize as the gelatinous enve- 
lope of the Frog and Toad, and the capsule of the Newts. These envelopes are not 
merely simple means of protection to the egg during the production of the embryo, 
as has been supposed, but, as I shall presently show, are essential to it at the period 
of fecundation, and without which the egg is not susceptible of impregnation. The 
layer of envelope which I regard as the foundation of the chorion, is a dense, but 
very transparent thin covering, in immediate contact with the vitelline membrane, 
and is formed of cells so closely aggregated together as to have coalesced into a 
fibrous structure. The two layers external to this are also formed of cells, which, 
with their nuclei, are distinctly visible in the envelope of Triton palustris, in which 
they alternate in regular series. Although these layers, which constitute the jelly in 
the egg of the Frog, become detached in that of the Newts quickly after oviposition, 
and, expanding as in the Frog, they leave the egg at liberty in a chamber in their 
interior, they are nevertheless essential to the impregnation of the ovum, which takes 
place before or at the time of leaving the body, as in Frogs and Toads. Rusconi* 
removed the envelopes of the egg of frogs, and found that the embryo still became 
developed, and thence concluded that these coverings serve only mechanical purposes 
during the changes ; but it will presently be seen that they have a more important 
function at a much earlier period. During the time they are in course of formation 
around the egg the yelk undergoes some further change. The light portion becomes 
of a whiter, and the dark portion of a deeper colour. Internally the cells vary more 
in size, the lighter-coloured being the largest. 1 have not succeeded in recognizing 
any embryo or central vesicle up to this period. 
2. CHANGES AFTER SPAWNING AND IMPREGNATION. 
First period of development. — It has been long known that a division or cleavage of 
the yelk of the Frog’s egg is one of the earliest and apparently invariable results of 
fecundation. The primary division was first seen by Swammerdam, and was figured-f- 
and mentioned, but was not understood by him. Spallanzani long afterwards re- 
cognized it in the egg of the Toad {Alytes obstetricans\) , which he says becomes about 
a day after fecundation marked “ with two furrows which meet to form an angle,” — 
that the furrows afterwards become deeper, and that “two small tumours arise on 
each side of the furrows,” — changes which have since been more accurately and com- 
pletely described by VoGT§. Spallanzani also says that the egg of the common Toad 
is “ marked with four furrows which intersect each other at right angles nearly like 
the husk of a chestnut half-opened,” but he seems to have thought this was the usual 
condition of the ovum. To Prevost and Dumas \\, however, we owe the important 
* Developpement de la Grenouille commune. Milan, 1826. 
't' Loc. cit. tab. xlviii. figs. 5, 8. f Loc. cit. vol. ii. p. 159. 
§ Untersuchungen fiber die Entwicklungsgeschichte der Geburtshelfer-Krsete {Alytes ohstetricans), 1842. 
II Annales des Sc. Nat. tom. ii. p. 110, 1824. 
