In some individuals the spermatozoa are so numerous that the intestinal canal appears completely' enveloped by them, and the whole peri-intestinal cavity seems alive with their movements.
In the mature ovary ova are discoverable in different degrees of development, in each of which the vesicles of Wagner and of Purkinje are, according to Professor Van Beneden, dis tinctly visible. In those ova which approach their complete maturity an external vitelline membrane, or chorion, and a vitellus are per ceptible, but the two vesicles above men tioned have disappeared.
When arrived at the proper term the ova break from their envelope, or ovisac, and fall into the general cavity of the body, where they move freely about surrounded on all sides by spermatozoa. At length the eggs accumulate in the interior of the body, near the base of the tentacula, and their escape, as witnessed by Van Beneden in Laguncula repens, is at length ac complished in the following manner. An egg presents itself at an orifice, situated in the vicinity of the anus, through which its external membrane partially protrudes, constituting a sort of hernia (fig. 61,p). The vitellus gra dually flows from the still enclosed portion of the egg into that which is external, and when the vitellus has thus entirely passed out, the egg is found separated from the parent animal and falls into the surrounding water. These eggs are entirely destitute of external cilia, and are carried off by any casual current to attach themselves where chance may bring them ; they are also remarkable for the irre gularity of their shape, some being completely angular, their form seeming to depend upon the pressure they have been subjected to in the interior of their parent.
Development by ova.— In Pedicellina Pro fessor Van Beneden has witnessed the escape of upwards of twenty eggs from a single indi vidual. They are of a pyriform figure, and enclosed in a pellucid membrane, by the in tervention of which they adhere together (fig. 66. 1), so that in the interior of the body of the parent Bryozoon they have a racemose appearance, and when extended spontaneously they are generally united together in pairs. Between the vitellus and the envelope of the egg there is always a small quantity of a trans parent whitish fluid, which doubtless repre sents the albumen, while the pellucid external membrane itself is the chorion.
The vitellus breaks up into granules, at first of large size, and afterwards by sub division of smaller and smaller dimensions, giving a tuberculated appearance like that of a raspberry to the mass. This division seems to be accomplished exactly as in the ova of the higher animals, the yolk first separating into two (fig. 66. 3), then into four (fig. 66. 2), after which its breaking up proceeds rapidly (fig. 65. 4).
The embryo enclosed within the egg soon assumes a rounded form and speedily appears divided by two indentations near its middle (fig. 66. 5), by which it is separated into an anterior and a posterior moiety, and vibratile cilia become apparent upon the anterior ex tremity.
That portion where the cilia have become apparent insensibly enlarges and assumes the shape of a funnel (fig. 66. 6), while the long cilia by which it is fringed begin to keep the particles suspended in the water around them in rapid motion. The margins of the funnel gradually extend themselves (fig. 66.1) the body exhibits frequent contractions, and at the end of about two hours little tubercles become apparent upon its anterior extremity, which subsequently become developed into the tentacula. Professor Van Beneden thinks that the original cilia disappear when the ten tacula have become developed and furnished with their proper vibratile apparatus. The formation of' the tentacula at once indicates which are the two extremities of the body and the point by which the embryo will subse quently attach itself.
The embryo when mature is quite at liberty and strikingly resembles some forms of Infu soria, but aftet a little while a pedicle is formed, whereby it proceeds to fix itself to some foreign body, and thus permanently assume the aspect of its race (fig. 66. 8). The pedicle seems to be formed by a cell developed below the stomach, which grows directly outwards, and thus completes the organisation of the young Bryozoon.
(T. Itymer Jones.)