THE ELASMOBRANCH FISHES 



307 



The lining of the uterus differs greatly in oviparous and viviparous types. 

 In the former it may be practically smooth or it may be thrown into low folds 

 as in Scyllium. In viviparous types it may be singularly modified as maturity 

 is reached. In an immature specimen of Squalus the lining is smoother than 

 that of the oviparous Scyllium, but in a specimen of Squahis which is pregnant 

 the whole surface of the lining is thrown into oblique rows of flaps or villi 

 (vi., fig, 267a), each of which is a leaf -like structure with exceedingly thin 

 walls. In a type like Torpedo or the butterfly ray, Pterojjlatea micrura, the 

 uterine wall may be thickly covered with long papillae some of which in the 

 latter may reach 10 to 20 mm. in length. The terminal part of such a papilla 

 is shown in figure 269b, in which it is seen that the wall is like a sponge. 



Fig. 268. Section through the uterus of MusteVus antarcticus. (From T. J. Parker.) 

 m., muscular layer; mu., mucous liuiug; p., peritoneal layer. 



Viviparous females injected during pregnancy show that the blood supply 

 to the uterus is exceedingly profuse. The arterial supply, in Acanthias, for 

 example, consists in part from anterior and in part from posterior oviducal 

 arteries which break up into branches to the oblique rows of villi. Each ter- 

 minal artery courses along the free border or fold of the row of villi (t.a., fig. 

 267a) supplying each individual villus with blood. If a cross-section be taken 

 through a single villus of Acanthias (fig. 267b) the finer vessels of the villus 

 may be made out. The large openings are for the terminal artery (t.a.) and 

 the central and smaller apertures are for the central veins which drain the 

 villus into a main uterine vein. Around the surface of the villus is the capillary 

 net (cp.) connecting the two systems. The same arterial arrangement is pres- 

 ent in Scymnus, although here a single villus is not so wide. 



KELATION OF UTEEUS TO EMBRYO 



The relation of the villi to the embryo is seen to advantage upon opening the 

 uterus of Acanthias. Here the villi come in close contact with the embryo and 

 multitudes of them are found clinging to the yolk sac on which the embryonic 

 blood system is profusely spread out. By the close relation of the embryo to 

 the maternal tissue an exchange between the two is insured. 



In Mustelus laevis the blood system of the yolk sac comes into still closer 

 relation with the walls of the uterus than in Acanthias. For here, branched 



