Page:Natural History Review (1861).djvu/61

Rh Leuokart, also, expresses himself in a very similar manner. Meissner, however, as is well known, has given a different, and very remarkable acconnt of the development of the eggs in Mermis. According to him, the eggs commence as a cell with a nucleus; the nucleus divides, and the new nuclei become the germinal vesicles, while the old cell-wall is gradually produced into follicles, into each one of which a germinal vesicle enters. Finally, the follicles are, by gradual constriction, separated from one another; and in this manner a whole festoon of eggs, besides several abortive follicles, originate directly from the modification of a single cell.

Sphærularia offers so many points of agreement with Mermis, that the development of the eggs naturally became specially interesting; and although my observations are very incomplete, I can ac least say, that, if the account given by Meissner is correct, there is in this respect, at least, no similarity between the two genera.

At the extreme end of the ovary I found a large cell with a nucleus. Following this cell are a great number of small vesicles, which much resemble true nucleated cells. They occupy the whole cavity of the ovary, and each of them is about of an inch in diameter. These are at first transparent, but gradually become more and more opaque on their inner side, from the deposition of minute yolk globules. The Purkinjean vesicle is also distinctly visible, but I could see no macula. As the ovary widens, the eggs gradually become wedge-shaped, the outer, larger portion remaining clear, so that in this part of the ovary there is a transparent border, with an opaque central axis. This axis, which is known under the name of "rachis," becomes gradually smaller and smaller, being absorbed into the growing egg, which becomes more and more opaque, and assumes a round shape, the Purkinjean vesicle remaining for some time visible in it, and containing a single macula.

When, however, it has entered the wide part of the tube, which we may probably call the uterus, it has again become elongated, and has lost the Purkinjean vesicle, and the yolk has begun to undergo segmentation. Pl. I., f 11, represents a very common state of the egg at the beginning of this process: the first two yolk-spheres, each with its nucleus, lie at the two extremities of the egg; and the central part is occupied by a mass of yolk, divided into an uncertain number of irregular masses, which however contain no nuclei, and are not regular spheres of segmentation. Farther down the uterus we find eggs in all stages of segmentation (Pl. I., f. 13); and in several instances I could distinctly see the nucleus dividing, as in Pl. I., f. 12, in preparation for the next division of the yolk. The segmentation is already far advanced when the egg is laid, but I never found in the uterus any eggs with a fully developed embryo.

I noticed a few specimens in which all the eggs near the vulva were broken up into irregular masses, and in one specimen this was even carried so far, that it began when the eggs were only about half grown. In normal eggs, the development of the young takes place in the manner usual among Nematoids.