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yet been replaced by a process of absorption from the uterine wall as 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, etc.). 



Amongst the Teleostean fishes viviparity occurs occasionally, 

 in at least half-a-dozen different families; the Cyprinodontidae, 

 Scorpaenidae and Embiotocidae furnishing the greatest number of 

 cases. They are particularly numerous amongst the Embiotocidae 

 and Scorpaenidae of the western coast of North America (Eigen- 

 mann, 1894). 



Fig. 218. — Portion of uterus of Ftero- 

 plataea mici'itra slit open to show an 

 embryo with the trophonemata (tr) pro- 

 jecting into its spiracles. (After Wood- 

 Mason and Alcock, 1891.) 



