Page:The Zoologist, 1st series, vol 1 (1843).djvu/235

Rh taken as an example. After the gemmule has become fixed and rooted, it becomes elongated superiorly, to the length of an internode, in a bulbous manner. One, and frequently several, of the lower internodes are destitute of cells; in such cases the pulp undergoes no further change. Superiorly, where the cells are formed in pairs, the elongation of the pulp takes place in the same manner, but is much larger and more bulbous, being of the length and thickness of the two cells. At this time it appears as a diffuse, semi-opaque, granular mass, without any distinction of parts. The first change observed is three darkish spots at the base, at equal distances, in a transverse line, answering to the two cells and the continuation of the stem. At first these spots have no defined edges, but are softened off into the surrounding pulp. As they become larger, they become deeper in tint, more defined, and assume the general form of the polype; the external surface of the pulp also becoming more and more translucent as development goes on. At first the sheath is nowhere apparent, but as the edges of the pulp become pale and translucent it becomes drawn from the sheath, and frequently into horizontal folds, as if from a force acting towards the centre. Condensation or organization thus proceeds from below upwards, and from the centre towards the circumference. After the pulp has become separated from the sheath its upper surface becomes serrated, and afterwards organized into the tentacula. The polype thus formed is drawn towards the centre of the cell, which remains closed, so that the polype is entirely included from the water. The upper portion of the cell, which eventually becomes the mouth, gets more and more attenuated, and finally ruptures; the ruptured edges fall in, and give the cell a funnel-shaped opening. If the bursting of the cell depended on pressure from within, it must be from the polype; but as the part, before it opens, does not bulge much, so when it ruptures, the edges, instead of being forced out, fall in; exactly the reverse of what would be expected if the rupture depended on internal pressure. In addition to this, it will generally be found that the polype does not touch the upper part of the cell for some time before it ruptures, and hence can have but little or no mechanical effect in forming the aperture. Though all the cells open in this manner, yet in some the mouth is formed above, and in others by the side; but the specific differences are prominently formed after all communication between the cell and polype has ceased, and after the cell is ruptured, and hence are probably produced from a power within the sheath itself. In a transverse section of the stem of a horny zoophyte it is found to be annular, the centre of the ring being