This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1233). When the cell has nearly reached its full development, its parietes become softened, and an opening is formed, which brings the young polyp into communication with the surrounding element. The Polyzoon has now attained its complete form, and can expand its tentacula; but, as yet, there are no traces of a generative apparatus, which seems to be matured at a subsequent period.
(1234). Reproduction is likewise effected, in the Polyzoa, by means of true ova. The ovary in which these are developed is situated immediately above the stomach, and is generally found containing eggs in different stages of growth. In the same vicinity is situated another viscus, regarded by Van Beneden as the testes, his opinion being founded on the fact that, when a mature specimen of the animal is placed between two plates of glass and gently compressed, so as to rupture its parietes and cause the escape of the viscera, spermatozoa are easily discoverable in its interior.
(1235). The spermatozoa exhibit considerable vivacity in their movements, have a disk-like body and a candal filament, and are proportionately of large size. Around them may be seen multitudes of free cellules without caudal appendages, which are apparently young spermatozoa. In some individuals the spermatozoa are so numerous that the intestinal canal appears completely enveloped by them, and the whole periintestinal cavity seems alive with their movements.
(1236). In the mature ovary may be seen ova in different states of development, in each of which the vesicles of Wagner and Purkinje are distinctly visible. In ova approaching their complete maturity, an external vitelline membrane, or chorion, and a vitellus are perceptible, but the two vesicles above-mentioned have disappeared.
(1237). When arrived at the proper term, the ova break from their envelope, or ovisac, and escape 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 parent, near the base of the tentacula; and their expulsion is ultimately accomplished in the following manner, through a special orifice in the immediate vicinity of the anus: - When an ovum is thus about to escape, its external membrane is first seen to protrude partially through the aperture, constituting a sort of hernia; the vitellus then gradually 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 irregularity of their shape, their form seeming to depend upon the pressure they have been subjected to in the interior of their parent.
(1238). In Pedicellina, Professor Van Beneden has witnessed the escape of upwards of twenty eggs from a single individual. They are of a pyriform shape, and are enclosed in a pellucid membrane, by the intervention of which they adhere to each other, so that, in the interior of the body of the parent Polyzoon, they have a racemose appearance, and when extruded spontaneously are generally united together in pairs. Between the vitellus and the envelope of the egg there is always a small quantity of a transparent whitish fluid, which doubtless represents the albumen, while the pellucid external membrane itself is the chorion.
(1239). The vitellus breaks up into granules, at first of large size, and afterwards, by subdivision, 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 yelk first separating into two, then into four, after which its breaking up proceeds-rapidly.
(1240). The embryo enclosed within the egg at first presents a rounded form, but soon becomes divided by an indentation into an anterior and posterior moiety, and vibratile cilia become apparent upon the anterior extremity. That portion upon which the cilia have made their appearance next insensibly enlarges, and assumes the shape of a funnel, while the long cilia with which it is fringed begin to keep the particles suspended in the water around in rapid motion. The margins of the funnel rapidly extend themselves; the body exhibits frequent contractions, and at the end of about two hours little tubercles are apparent upon its anterior extremity, which subsequently become developed into the tentacula. Professor Van Beneden thinks that when the tentacula have become developed and furnished with their proper vibratile apparatus, the original cilia disappear. The formation of the tentacula at once indicates which are the two extremities of the body, and the point by which the embryo will subsequently attach itself.
(1241). The embryo, when mature, is quite free, and strikingly resembles some forms of Infusoria; but after a while a pedicle is formed, whereby it proceeds to fix itself to some foreign body, and thus permanently assumes the aspect of its race. The pedicle seems to be formed from a cell, developed below the stomach, which grows directly outwards, and thus completes the organization of the young Polyzoon.
(1242). A third form of reproduction is that by ciliated gemmules, common in Halodactylus diaphanus and other similar species having soft and fleshy or gelatinous poly-paries. These are readily seen in spring, when they appear as minute whitish points imbedded in the substance of the mass; sometimes, however, they are of a dark-brown colour, and exceedingly numerous, appearing to occupy almost the entire substance of the po-lypary (fig. 240.) If one of these points bo carefully turned out with a needle and examined, it is found to consist of a transparent sac, in which are contained generally from four to six of the gemmules, which, as soon as the sac is torn, escape, and swim about with the greatest vivacity*. Sometimes they simply rotate upon their axis, or they tumble over and over; or, selecting a fixed point, they whirl round it in rapid circles, carrying every loose particle with them; others creep along the bottom of the watch-glass upon one end, with a waddling gait; but generally, after a few hours, all motion ceases, and they are found to have attached themselves to the bottom of the glass. At the expiration of forty-eight hours, the rudiments of a cell are observable, extending beyond the margin of the body; but any account of their further development is still a desideratum.