. Text-book of embryology. Embryology. VIII VIVIPARITY IN FISHES 479 yet been replaced by a process of absorption from the uterine wall as is the case in the Mammalia. But the uterine wall already plays a part though a minor one in providing food material for the young individual by its glandular activity. The beginnings of this are seen in the albuminous fluid enclosed within the egg-shell, and it is possible that the elongated gill-filaments of the embryo play a part in absorbing nourishment from this. A further development consists in the secretion of an abundant " uterine milk "
. Text-book of embryology. Embryology. VIII VIVIPARITY IN FISHES 479 yet been replaced by a process of absorption from the uterine wall as is the case in the Mammalia. But the uterine wall already plays a part though a minor one in providing food material for the young individual by its glandular activity. The beginnings of this are seen in the albuminous fluid enclosed within the egg-shell, and it is possible that the elongated gill-filaments of the embryo play a part in absorbing nourishment from this. A further development consists in the secretion of an abundant " uterine milk " which is drawn into the pharynx through the spiracles by precociously occurring move- ments like those of respiration and passed on into the digestive tract. In accordance with its glandu- lar activity the lining of the uterus frequently undergoes an increase of area by growing out into villi or trophonemata (Wood-Mason and Alcock, 1891). In the Sting- Rays specially enlarged tropho- nemata may be drawn into the pharyngeal cavity of the embryo through its greatly dilated spiracles so that their secretion reaches the alimentary canal of the young fish directly (Fig. 218). During the later stages of intra- uterine development there usually comes about an intimate relation- ship between the surface of the yolk-sac and that of the uterine lining and in association with this there is found a varying degree of specialization of the uterine lin- ing (Ercolani, 1879; Widakowich, 1907). This latter may be smooth (Squatina angelus, Notidanus cine- reus), or project into longitudinal folds so as to give increase of surface (Acanthias vulgaris, Scymnus lichia), or grow out into papillae or trophonemata {Torpedo, Fteroplataea). Or finally it,may develop folds which interlock with grooves on the surface of the yolk-sac, the uterine and yolk-sac surfaces being in the most intimate contact so as to constitute physiologically a definite yolk- sac placenta (Carcharias glaucus, Mustelus laevis, e
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