Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/87

GERMINAL MEMBRANE brane, that is, from such cells as are met with in the area pellucida, so that the embryo is composed, partly of small cells without nuclei, and partly of cells furnished with the characteristic nucleus. It presents, however, besides them, an extraordinary quantity of simple cell-nuclei with nucleoli, around which no cells have as yet formed.

I have made but few researches with respect to the structure of the vascular layer, and from them, I could not (with the exception of the vessels themselves and the blood) detect any such essential difference between it and the mucous layer, as was exhibited between the latter and the serous layer. As, however, the formation of the vessels themselves, although it ap- pears to depend upon a production of cells, is not a process peculiar to the germinal membrane, we shall defer it, to be resumed at a subsequent stage of our investigation.

I have not ascertained the relation which these cells of the layers of the germinal membrane have to the primitive globules of the membrane before incubation, or within eight hours after that process has commenced; but inasmuch as it is probable that at least one of those kinds of cells owes its origin to the development of the primitive globules, we may be permitted to suppose that those globules are likewise cells.

For the purpose of giving, in outline, a connected view of the changes which the egg undergoes, from its first formation up to the period at which the actual development of the embryo commences,—in so far as the foregoing, more or less complete, observations enable us to form a provisional conception of the process of development,—we will proceed on the understanding, that the germ-vesicle is the nucleus of the yelk-cell ; at the same time, however, we expressly refer the reader to the more detailed statement above furnished for the certainty both of this and of every other separate point which occurs in the following exposition. It is probable that the germ-vesicle is the first structure, and that the yelk-cell forms around it as its cell-nucleus. Both advance in growth, the latter, however, much more rapidly than the former. A precipitate, the commencement of the germinal membrane, next forms around the germ-vesicle. Young cells are simultaneously formed in the remaining space of the yelk-cell, these are the cells of the subsequent yelk-cavity. Then cells of another kind originate beneath the vitelline membrane,