(244). In the earlier stages of growth the polypary consists of a primary trunk, from which alternating pedicles are given off at regular distances. These pedicles soon become transformed into branches, on which new pedicles in turn make their appearance, as they did on the original stem, exhibiting a dichotomous or trichotomous arrangement.

(245). At the base of each branch transverse rings are formed (fig. 47, g), which are persistent during the life of the polypary. All the branches, as well as the common trunk, increase in their dimensions in accordance with the age of the zoophyte; and, as in vegetables, there is a relation preserved between the thickness of the trunk and the number and extent of the branches.

(246). Each polyp-bearing cell at the extremities of the branches presents externally a bell-shaped cup, having at its bottom a horny diaphragm, perforated in the centre. It is through this perforation that the body of the polyp is brought into communication with the common fleshy substance of the polypary, and through its intermedium, with the other polyps.

(247). From the recent observations of Van Beneden* relative to the embryogeny of the Campanularian polyps, it would appear that they frequently undergo, during their development, a series of changes not less wonderful than those exhibited by the other Hydriform races whose history has been carefully traced.

* Nouveaux Mem. de l'Acad. de Bruxelles, vol. xvii. (1843).

(248). The ovum developed in the ovarian vesicle is, at its first appearance, of a spherical shape, and imbedded in the substance wherewith the deciduous capsule is filled (fig.

47, e, e'.) It is surrounded by a membrane analogous to the calyx of the ovary in buds, which, being torn, discovers the denuded ovum, wherein the vesicles of Pur-kinje and of "Wagner are easily detected; but these very speedily disappear, without any other change in the interior of the egg being discernible.

(249). The next step in. the process of development seems to be the conversion of the external vitelline cells into a layer, situated immediately beneath the vitelline membrane, which may be regarded as the representative of the blastoderm.

(250). The blastoderm now becomes thickened around the vitellus, forming a sort of elevated ring (fig.

48, b), and the positions of the different organs hereafter to be developed become recognizable.

(251). Certain cells now begin to make their appearance in the interior of the blastoderm, the arrangement of which is particularly remarkable (fig. 48, c & d, b); these cells arrange themselves in groups of five around the circumference of the blastoderm, and have the appearance of so many crystals: the form of each group is quadrilateral; but subsequently, at each angle there is developed another cell, connecting the two groups together, and making the whole number to amount to twenty-four. These twenty-four cells will afterwards become the tentacles of the polyp.

Campanularia gelaiinosa. a a, tegumentary skeleton, or horny polypary.

Fig. 47. Campanularia gelaiinosa. a a, tegumentary skeleton, or horny-polypary; b, b', buds in progress of development into polyps; c c c, terminal polyp-cells empty; d,d',d",d'", polyps in different stages of growth; e, ovarian cell containing an embryo ready to escape; e\ another ovarian cell containing several embryos in various states of development; f, living substance filling the interior of the horny polypary; g, annular constrictions of the horny skeleton. (After Van Beneden).

Embryology of Campanularia gelatinosa.

Fig. 48. Embryology of Campanularia gelatinosa. A, an ovarian vesicle from which an embryo is in the act of escaping; others, in a less advanced state, are seen in the interior. B, a detached embryo in a very early stage, showing the vitellus and blastoderm. C, another embryo, more highly magnified: b, cells formed around the vitellus. D, the same, more advanced: a, cavity enclosing the remains of the vitellus' 6, elongated cells subsequently developed into tentacula; c, other cells, eight in number, which are the rudiments of the organs of the senses. E, the same in a more advanced condition. E, an embryo at the moment of its escape from the ovarian vesicle, magnified: a, fleshy pedicle; b, mouth; d, muscular fasciculus; e, nervous ganglion; ff, organs of sense; g, tentacles. G, the same, as it swims in the water, presenting all the characters of a Medusa.

(252). Cells of another order now make their appearance (fig. 48, d, c), grouped together in pairs, behind the preceding, with similar regularity. These will become organs of sense.

(253). It is difficult to avoid making the comparison between the above appearances and those of crystallization; the cells, in fact, arrange themselves precisely like crystals, with perfect symmetry, and always in accordance with the number four or its multiple.

(254). The embryo, at this period of its development, presents the shape of a thick lens-like disk; and shortly, from the centre of its inferior surface, there is developed a tubercle, destined hereafter to become the body of the polyp; it is by this part that it will ultimately become attached.

(255). The four cells formed between the individual groups above mentioned are, in this stage of the growth of the little being, so compressed that they seem to be quite lost; soon, however, they expand so as to press upon their neighbours, and then the disk appears to be surrounded with a regular series of cellules, twenty-four in number, which, as they become developed, shoot out externally, and soon present themselves under the appearance of so many tubercles (fig. 48, e).

(256). The eight interior cells (c c) take another direction, but their form remains unchanged; and they exhibit, up to the termination of this embryo condition of the animal, a nucleus in the centre of each, which might be regarded either as a crystalline lens or an otolith, according as these organs are judged to be eyes or auditory capsules; for such are the designations applied to them by modern zootomists, as will be explained in the next chapter.