The structure of the duct and of its numerous ovisacs is best seen in those specimens that have not yet arrived at maturity, or in those which have just deposited one laying of eggs. In these individuals the oviduct, to within a short distance of its division into two outlets, is studded with minute ovisacs, each filled with the rudiments of its minute ovum. Its ge neral appearance in a female, that has recently deposited its eggs, is completely botruoidal, very like the ovary of Birds, some ova being always fully developed, and ready to pass into the oviduct, while others are in various stages of developement, many of which are imper ceptible except with the aid of a powerful lens.
But the most remarkable condition of the female organs in the Julidae is their double vaginal outlet, as in Crustacea, although the oviduct itself is a single tube until near its termination, where it is divided into two short canals, which from a slight opacity at their base, where they join the single duct, appear, when seen by transmitted light, to be separated from it by a valve or duplicature of the lining mucous membrane. The vaginal orifices are simply two nipple-shaped portions of the tegu ment, with somewhat oval apertures sur rounded by a corneotis ring, from which is developed a circle of minute hairs. They are situated on the under surface of the fourth segment of the body, and correspond in posi tion to the insertion of the legs in the third segment.
Ova.—We have already seen from Mr. New port's description of the female generative system of Julus, that the ova are formed in separate ovisacs, from which they issue com pletely constituted eggs, into the cavity of the ovarium or common duct, through which they are expelled from the body after impregnation ; and we now proceed to lay before the reader the important results of the investigations of that distinguished anatomist relative to the struc ture of the ova themselves, and the progress of embryonic developement. The existence of the ovisacs in Julus as single isolated cap sules on the exterior of the oviduct, in each of which a single egg is produced, is, Mr. Newport observes, a circumstance particularly favourable to a minute examination of the ovum in all its states, especially as ova are found at the same time in every stage of de velopenient. The smallest ()view's appeared like very minute glandiform bodies, developed, as it were, immediately from the structure of the duct itself, and in these the rudiments of the future egg had already begun to be pro duced. The smallest rudiments of eggs ex amined were of an elongated shape, and as yet not more than three, or at most four blood globules in diameter. They appeared already
to have distinct parietes, and to be filled with very minute graniform cells of a uniform size, slightly opaque, and of a yellow colour. The diameter of these cells, as nearly as could be ascertained by direct comparison, was equal to about one-third of that of a blood-globule. In the midst of these cells there was a larger but much more delicate structure of a circular form and equal in size to about two of the cells, but whether this was the germinal vesicle or its macula could not be determined. Other ovi sacs twice the size of the foregoing were tilled with similar contents, and from the opacity and yellow colour of the graniform cells, it was evident that they constituted the yelk in one of its earliest stages. At a later period both the yelk and its including vesicle are in closed in a distinct membrane—the membrane vitelli, and before its escape into the oviduct all the parts of a perfect egg, namely, the yelk, the germinal vesicle with its macula, the mem brane vitelli, the albumen, and likewise the shell lined by the membrane externa or chorion, are completely formed.
Evolution of the embryo.—The develope ment of the young Julus Mr. Newport di vides into several distinct and well-marked pe riods, during each of which phenomena are pre seined of the utmost interest, both to the phy siologist and in an entomological point of view.
The first period extends from the deposition of the egg to the gradual bursting of the shell, and exposure of the embryo within it, occu pying the space of twenty-five entire days, during which the egg acquires a sensible in crease of bulk.
On the nineteenth day there was a complete alteration in its form ; it was more obtuse at both ends, and had become much larger, and the outline of the embryo, coiled up within the shell and nearly filling the whole interior, was very distinct, although, as yet, there were no rudiments of limbs or even of a division of the body into distinct segments. On the fol lowing day, the twentieth, the outline of the embryo was more apparent, and on its concave or ventral surface there were faint traces of a di vision of the body into six segments (fig. Up to this period Mr. Newport was unable to detect any funis or umbilical cord attached to the embryo, although, in conse quence of Rathke's observations in Crustacea, such a structure was particularly sought for, the whole body still appearing to be formed of cells of different sizes.