The Embryonic Attachment (“Nabelschnur”).

This structure, as already indicated, is an outgrowth of the wall of the bursa, from the position at which the embryo at first becomes adpressed to it, after liberation of the egg from the ovary—that is, from the abradial wall. With the growth of the embryo the portion of the bursal wall immediately related to it begins to grow out, to form an elongate stalk-like organ with the embryo at its distal extremity. One is reminded of the analogous “placenta” by which the gametophyte of an angiosperm plant is attached to the wall of the seed-capsule formed by the parent sporophyte. In the figure of the fully-developed larva (Fig. 12) this organ is also seen at its greatest development. In section its structure proves to be very simple (see Fig. 10, Emb.At.), comprising only a tissue of undifferentiated cells. These are somewhat spindle-shaped, elongate and staining only lightly with cytoplasm dyes. There are no sinuses or other vascular structures present in the organ, and there is little but superficial resemblance to an umbilical cord. After the assumption by the embryo of radial symmetry (see below), the attachment survives for a while, but becomes shorter again. It is then attached to one of the interradii of the young star, towards the dorsal side (Fig. 20). After the completion of the “pentagon” stage of the young ophiuroid, the attachment atrophies, the embryo breaking off and lying freely in the lumen of the bursa. A young free embryo is sometimes found in the bursa with the stump of the embryonic attachment still projecting from one interradius.