. The elasmobranch fishes. Chondrichthyes. THE ELASMOBRANCII FISHES 303 urogenital. The urogenital sinus, as in Heplnuchus, then emi)ties by means of a urogenital ])ai)illa into the cloaca. Leigli-Sharpe (1920-21) has described fully the sii)lions of a number of Elasmobranclis. These are composed of longer or shorter closed sacs which end by siphon tubes. In a type like Acanthias the sac lies under the skin ventral to the base of the pelvic fin, and its tube empties into the proximal part of the clasper tube. In a large specimen of Galiorhinus this siphon sac extended almost to th
. The elasmobranch fishes. Chondrichthyes. THE ELASMOBRANCII FISHES 303 urogenital. The urogenital sinus, as in Heplnuchus, then emi)ties by means of a urogenital ])ai)illa into the cloaca. Leigli-Sharpe (1920-21) has described fully the sii)lions of a number of Elasmobranclis. These are composed of longer or shorter closed sacs which end by siphon tubes. In a type like Acanthias the sac lies under the skin ventral to the base of the pelvic fin, and its tube empties into the proximal part of the clasper tube. In a large specimen of Galiorhinus this siphon sac extended almost to the pectoral girdle. The walls of the sac are muscular and its function appears to be the forcing of the sperm cells through the clasper groove. Glands may line the whole sac as in Lamna or the dorsal side of its wall only (see ,fig. 31). The func- tion of the clasper gland in the latter condition is not definitely known. GENITAL ORGANS OF FEMALE OVARIES. Fig. 262. Section through shell gland, Scyl- Hum. (From Borcea.) , secretory cells. The ovaries of the adult female usu- ally arise as paired structures, and are bound to the anterodorsal wall of the body cavity by a mesentery, the mesovarium (Squalus, fig. 253a). Not infrequently, however, the left ovary atrophies in the adult (Scyllium, Prisfiophorus, Carcharias, Galeus, Mustelus, and Zygaena). They occupy the anterior part of a mass of tissue which, as the epigonal organ, may extend along the dorsal wall of the body cavity pos- teriorly where it joins the rectal gland. In numerous forms, however, the epigonal organ is wanting as, for example, in Acanthias. The ovary varies greatly, depending upon the stage of maturity of the ova contained. It appears as a sac through the walls of which the ova may be seen varying in size from relatively minute spots to mature ova often from 3 to 5 cm. in diameter (see 01'., fig. 252). In development an indifferent sex cell divides several times forming oogonia. Each oogonium then unde
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