The Reproductive Organs.

The morphology of the reproductive organs of Amphipholis squamata follows the general pattern of the whole genus, and the arrangement of the gonads can scarcely be distinguished from the condition described by Mortensen for A. tenuispina, A. japonica, and A. sabrina (Mortensen, 1920). As, however, the relationship of the gonads to the bursae is of great importance to understanding the subsequent history of the embryo, it is necessary to give here a somewhat more complete account.

There are ten bursae, so distributed that there are two in each of the five interradii. Each opens to the exterior through the narrow genital clefts, which lie on the under surface of the disc, one on either side of the base of each arm. Each cleft is bordered by two skeletal plates, the abradial and adradial genital plates respectively (Fig. 1). Attached to the dorsal (aboral) surface of each adradial genital plate is a single testis, thus making ten testes in all; the ten ovaries are each similarly related to an abradial genital plate. Occasionally a two-lobed testis is seen, or, less frequently, two testes are found attached to a single adradial plate. Occasionally, also, the ovaries are found to be secondarily increased in number in this way. The ovaries occupy a somewhat more peripheral position than do the testes, and thus when a series of vertical sections is being examined, the testes of any particular sector are seen cut across in sections nearer the centre than those in which the ovaries are cut.

As the relationship of the gonads to the bursa and neighbouring organs can best be understood by means of vertical sections, two examples are illustrated (Figs. 2A and 2B). In Fig. 2A is seen part of a vertical section through an interradius, including a bursa. The ovary of this bursa is seen to the right, on the abradial border of the genital cleft. To the left of the bursa is the basal part of the neighbouring arm, cut in vertical transverse section. The part of the bursa which is cut across does not show any portion of a contained embryo, but the presence of the sinuses in its wall indicate that there is an embryo present in another part of the organ (see section dealing with the bursa, and nutrition of the embryo, below). The ovary, which contains a small number of oogonia in various stages of maturity, is ensheathed by a thin wall one cell in thickness. That side of the ovary toward the bursa is closely adpressed against the latter, so that it would not be difficult for a ripe egg to be liberated through the bursa directly into the lumen. Further details in regard to the ovary are given in the section dealing with the ovum and oocyte.

In Fig. 2B is seen a similar vertical section from the same series, but differing in that it has been cut somewhat further in toward the centre of the disc, and thus the ovary is absent from the abradial side, while the testis is visible on the adradial border. There is also seen the testis belonging to the interradius on the other side of the arm. As the bursa is cut more centrally, an arm of a contained embryo is seen cut in transverse section. The testis, like the ovary, is very simple, comprising only a spherical capsule one or two cells thick containing loosely-arranged spermatogonia. Each of the latter is a spherical cell with a very large and deeply-staining nucleus. It is