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. I 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-}- 

 and mentioned, but was not understood by him. SPALLANZANI long afterwards re- 

 cognized it in the egg of the Toad (Alytes obstetricansty, 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. 



f Loc. cit. tab. xlviii. figs. 5, 8. J Loc. cit. vol. ii. p. 159. 



Untersuchungen iiber die Entwicklungsgeschichte der Geburtshelfer-Krsete (Alytes obstetricans), 1 842. 



H Annales des Sc. Nat. torn. ii. p. 110, 1824. 



