The nature of the change which confers upon the egg the power of production is entirely unknown to us,ancl we already remarked towards the commencement of this article that this action is to be ranked among those vital operations of the animal economy which are placed beyond the reach of our means of investigation. We should with equal prospect of success proceed to inquire how life originates and is maintained in the parent, as to investigate the secret man ner of the transmission of the vital spark from the parent to the offspring. The physiologist who would study this subject must therefore limit his inquiries in this as in other departments of his science to the search after those condi tions or chain of circumstances which appear to be essential to the occurrence of the parti cular change or phenomenon which is the object of his investigation.
Our present object, then, is not to investigate the nature of the change by which the living productive power is given to the egg, but to endeavour to establish what are the essential conditions of fecundation.
Difference between the fecundated and wife cundated ovum.—In the first place, in reference to this subject, it would be interesting to know any material difference exists between the structure of the fecundated and unfecun dated egg.
In the common fowl we have seen that the whole substance of the egg, the yolk and ger minal portion, the albumen, shell, and mem brane, may be formed in the ovaries and oviduct, and excreted from the body of the hen without any connection with the cock ; but such an egg, though apparently the same in structure with that which is laid after connection with the cock, when subjected to the requisite heat, undergoes none of the changes of development which incubation induces in the fecundated egg, but only passes into chemical decomposi tion like any other dead animal substance. Did any difference of structure exist between the fecundated and unfecundated egg, we should be disposed to look for it first in that part of the egg which is more immediately con nected with the new being, viz. in its germinal portion ; but we regret to say that the investiga tions of naturalists have not as yet pointed out any marked difference in a satisfactory manner. AIalpighi, it is true, long ago pointed out a difference in the structure of the cicatricula of the egg of the common fowl which had had connection with the cock, and tbose of the hen living single, and the observations of this author were afterwards confirmed by Prevost and Dumas. In the impregnated egg the cicatri
cula is a well-defined whitish spot, with a re gularly formed transPlrent area in its centre ; while in the unimpregnated egg there is no re gularly shaped transparent area, but rather a number of small irregular clear spaces scattered over the surface of the cicatricula. We fear, however, that this appearance of irregularity exists as well in some eggs that have been laid after connection with the cock, and that the shape or appearance of the cicatricula can scarcely be depended upon as informing us whether an egg has been fecundated or not, since that appearance is much influenced by the state of the nucleus or white matter of the yolk situated below it, as well as by the state of the cicatricula itself. This subject is worthy, however, of the most accurate investigation, as it appears to offer the prospect of affording some information on this very obscure part of the generative process.
In the ovarian ovulum, the vesicle of Pur kinje, it will be recollected, occupies the centre of the cicatricula ; and this vesicle exists in the ovulum so long as it remains within the ovarian capsule, whether the hen have connection with the cock or not. In the impregnated fowl the germinal vesicle of Purkinje bursts, and leaves the transparent area in the centre of the cicatri cula at the time when the ovulum passes from the ovarian capsule into the oviduct ; but it re mains to be known if the same is the case, or what phenomena ensue upon the escape of the ovulum from the ovarium in the fowl which is entirely separated from the cock.* We do not know with certainty what befalls the vesicle of Purkinje in the ovulum of Nam malia at the time of its escape from the ovarium. The analogy of all oviparous animals is strongly in favour of the supposition that it bursts in the same manner. Al. Coste states that it does not burst, and Valentin supports an opposite view.
While, therefore, we feel disposed to adopt the opinion that the seminal fluid, in fecunda ting the egg, operates its peculiar change chiefly on the germinal part, and that the bursting of the germinal vesicle is very probably connected with the change of fecundation, it must be ad mitted that further observations are still want ing to afford a satisfactory proof of the correct ness of these hypotheses.