In like rnanner when a stem is cut off trans versely, a bud is developed from the cut ex tremity, which by its growth prolongs the original trunk. When this kind of gemma has attained to a sufficient size there arises from its extremity a little crown of tubercles, and subsequently a second becomes manifest at some distance from the first ; and as the growth of these tubercles continues, each of them becomes at length developed into a ten tacle. The tentacle, therefore, grows from the body exactly in the same way as the bud from the stem, the only difference being that the for mer is solid, and the latter tubular.
The growth of the horny polypary exactly keeps pace with the development of the soft substance, and even goes beyond it. Below the tentacula the body soon becomes con stricted, marking the boundary between it and the stem; and soon the polype, becoming too large to be contained in its sheath, issues forth, and expanding its tentacula becomes perfectly unfolded. The oviferous pedicles, hereafter to be described, are developed sub sequ ently.
Second mode of propagation, by free gentince. —The free gemmw are produced upon dis tinct pedicles, which in the genus Tubularia are developed within the lower circle of ten tacula. They resemble numerous appendages disposed in a circle and forming a crown around the body of' the polype. (Fig. 48, o.) These pedicles grow in the same manner as the buds and the tentacula described above, that is to say, a hollow tubercle first makes its appearance, which seems to be merely an extension of the external covering of the polype. Each tubercle slowly expands, and soon divides into one or more branches, which are all hollow, and the same fluid which circulates in the general substance of the polype may be observed to pass into their interior.
At the free extremity of each of the pedicles thus formed a distinct cell is soon perceptible, situated im mediately beneath the surface, which cell is the rudiment of a new in dividual. (Fig.49, 1,a.) No nucleus has been remarked in its interior. This primitive cell, which might also be regarded as an egg or as an ovule, sometimes becomes organised internally, in which case the repro ductive process assumes the third or the fourth form, subsequently to be noticed, or else it serves for the point of departure, or it rnight almost be said the mould for the formation of a free gemma, which becomes organised around it at the expense of the pedicle itself. It is
in effect a part of the reproductive appendage that will subsequently become detached ; but at this period of its development it is impossible to determine after which of the four modes of reproduction the embryo will be formed. The vesicle (a) now increases rapidly in size, and beneath it another membrane is soon perceptible, which by its inner surface is in contact with the cir culating fluid. This membrane is the origin of the new individual, or, in other words, a blastoderm, formed by the internal skin, and not by the vitellus. Soon there is seen, projecting from its cen tre,a little cone (fig.19. 3, 4), which, compressing the vesicle (a), forms a depression upon its inner surface, and the vesicle now begins to assume the appearance of a serous membrane, yielding to the pressure of the organs over which it spreads, and ultimately covers, much in the same way as the pleura covers the lungs. The tubercle (b) will afterwards form the walls of the digestive cavity, and may be seen to have the circulating fluid derived from the body of the polype moving in its substance. Around the base of the cone (b) may now be seen four other tubercles (c, 4, et seq.), which become de veloped like the preceding; but, instead of compressing the vesicle (a), they surround it, and ultimately completely enclose it. They carry the skin with them, so as to have the appearance of a transparent vase, having four longitudinal prorainent bands, the free edge slightly enlarged and rounded, a pedicle in the middle like the stem of the vase, and the transparent vesicle lining its interior through out.
The different phases of the development above described will, however, be best under stood by a reference to the series of figures which we have appended, carefully copied from Professor Van Beneden's elaborate illus trations.
The young Tubularia has now assumed the appearance of a Beroe, and in this condition has doubtless been often mistaken for an in dividual belonging to the class Acalephm ; and lively contractions of its body are frequently witnessed, although it still remains attached to its pedicle.