The smallest gemmm are described by Dr. Farre as homogeneous in their texture, form ing little nodules on the parent stem. Those further advanced were seen to present some thing like a boundary line, indicating the thickness of the parietes of the future cell. Within this, in others, was a dark mass, which in larger ones presented a rough outline of the form of the complete animal. Those about half grown had all the parts distinctly traced out ; the retractor muscles completely formed ; the tentacles short and clumsy ; the walls of the alimentary canal thick, and its boundaries clearly defined.
This tnode of propagation has been still more completely studied by Professor Van Beneden, whose opportunities of observation enabled him to prosecute the inquiry more closely.
In Pedicellina the phenomena attending the gemmiparous mode of reproduction are de scribed by Professor Van Beneden as present ing,th e following phases of development. First, there sprouts from the common stem of the Bryozoon, without any determinate situation, a tubercle which is but a prolongation from the stem itself (fig. 65, a, 8); this tubercle extends outwards, becomes more prominent, and soon swells out into a vesicle (b, 8), which is- the first appearance of the new individual. Tip to this period the interior of this vesicle, is like that of the stem itself, of which it is only an extension; but now a cellnle becomes visible in its centre, which forms the point of departure whence the development of the embryo proceeds.
Around this primitive cell a series of other very small cellules soon group themselves, which seem to constitute the parietes of the primitive vesicle or the blastoderm, the original cell representing the vitelline cavity. The bud enlarges, and as its growth proceeds the internal tissue becomes thickened, so as to fill it ; subsequently an indentation is appa rent on each side of the little cavity which separates it into two halves, the inferior of which will form the stomach, properly so called, a hile the upper division will become the anterior space between the tentacula.
The mode of reproduction by gemmm has been carefully studied in the genus Laguncula (Lagenella of Farre) by the same investigator. The reproductive buds sprout from the creep. ing stems ( fig. 61, y) Which connect the indi vidual animals, appearing at first as a slight prominence, that soon expands into a rounded tubercle which is the commencement of a new cell'.
On close inspection, this bud is found to consist of a transparent envelope, which is. in fact, a continuation of the general invest ment of the polype. This rudimentary cell is lined throughout with a soft membrane, having its inner surface studded with minute globules, by the accumulation of which the polype is ultitnately formed. The bud itself is hollow, and communicates with the parent stem. It therefore has nothing in its composition re sembling that of an egg ; neither distinct vesi cle nor vitellus ; this condition of the gemma is represented in fig. 62. 1. The new formed cell soon grows taller, and its lining rnembrane becomes thicker, and indicates the commencement of the intestinal canal, which is at first a simple cavity, bounded by the thickened lining membrane of the cell. This cavity once formed, the development of the different organs proceeds rapidly. First, in the middle of the cavity there appears a longi tudinal fold resembling two lips (fig. 62.2), which, as they approach each other, divide the cavity of the body into an anterior and a posterior compartment. The two lips, vvhich have a valvular appearance, become indented very regularly along their margins, and are soon recognisable as the rudiinents of the tentacular circle (fig. 62. 3).
At this epoch, it must he remarked, the polype presents two cavities distinct from each other. There is a space between the walls of the body and the parietes of the future alimentary canal, the interspace being in communication with the stem of the parent polype, and filled with a fluid that is analogous to the blood of the higher animals; superiorly this cavity likewise penetrates into the ten tacles, and the fluid which bathes the exterior of the alimentary canal thus finds admission even to the extremities of those organs (fig. 62. 6, m).
first rudiments, the cavity of the stomach and the rest of the intestinal tube gradually be come apparent, and at the same dine some globules are visible disposed around the cul de-sac of the former viscus, which gradually become arranged into fibrillm, and constitute the retractor muscles.