o-i INTERNAL YOLK-SACK. 



proximal end of the subintestinal vein becomes the portal vein, and 

 it is joined just as it enters the liver by the venous trunk from the 

 yolk-sack. The venous trunk leaves the body on the right side, and 

 the arterial on the left. 



The yolk-sack persists during the whole of embryonic life, and in 

 the majority of Elasmobranch embryos there arises within the body 

 walls an outgrowth from the umbilical canal into which a large 

 amount of the yolk passes. This outgrowth forms an internal yolk- 

 sack. In Mustelus vulgaris the internal yolk-sack is very small, and 

 in Mustelus Itevis it is absent. The latter species, which is one of 

 those in which development takes place within the uterus, presents 

 a remarkable peculiarity in that the vascular surface of the yolk-sack 

 becomes raised into a number of folds, which fit into corresponding 

 depressions in the vascular walls of the uterus. The yolk-sack 

 becomes in this way firmly attached to the walls of the uterus, and 

 the two together constitute a kind of placenta. A similar placenta 

 is found in Carcharias. 



After the embryo is hatched or born, as the case may be, the yolk- 

 sack becomes rapidly absorbed. 



BIBLIOGRAPHY. 



(40) F. il. Balfour. "A preliminary account of the development of the Elasmo- 

 brauch Fishes." (Juart. J. of Mier. Science, Vol. xiv. 1876. 



(41) F. M. Balfour. "A Monograph on the development of Elasmobranch 

 Fishes." London, 1878. Reprinted from the Journal of A nut. and Pliysiol. for 1870, 

 1377, and 1878. 



(42) Z. Cierbe. " JiecJierches .turln segmentation de la clcatrule et la formation des 

 jirudtiits adrentif.s de Vu'iif <!,'.; J'ln^iiixtiinH'n i't particitlieremeitt den Haics." Vide also 

 Journal de VAnatomie et de In Physiologie, 1872. 



(43) W- His. "Ueb. d. Bildung v. Haiiischeuembryoneu." Zcit. fiir Anat. u. 

 Entwick., Vol. n. 1877. 



(44) A. Kowalevsky. "Development of Acanthias vulgaris and Mustelus lasvis." 

 (Russian.) Transactions of tin' Klew Society of Naturalists, Vol. i. 1870.. 



(45) R. Leuckart. "Ueber die allmiihlige Bilduug d. Korpergestalt bei d. 

 Rochen." Zeit.f. u-iss. Zool., Bd. n., p. 258. 



(46) Fr. Leydig. Roche it u. Haie. Leipzig, 1852. 



(47) A. W. Malm. "Bidrag till kanuedom om utvecklingen af Rajae." Konjl. 

 cetenskaws akademiens forhandlingar. Stockholm, 1876. 



(48) Joh. Miiller. Clutter Haie des Aristoteles nnd iiber die Verschiedenfreiten 

 it nter den Haijischen und liocheii in der Entwicklunti des Eien. Berlin, 1.S40. 



(49) S..L. Schenk. "Die Eier von Raja quadrimaculata iunerhalb der Eileiter." 

 Xitz. der k. Akad. Wicn, Vol. LXXIII. 1873. 



(50) Alex. Schultz. " Zur Entwicklungsgeschichte des Selachiereies." Arclnv 

 fiir micro. Anat., Vol. xi. 1875. 



(51) Alex. Schultz. " Beitrag zur Entwicklungsgeschichte d. Knorpelfische." 

 Arch if fiir micro. Anat., Vol. xm. 1877. 



(52) C. Semper. "Die Stamrnesverwandschaft d. Wirbelthiere u. Wirbellosen." 

 Arbeit, a. d. zool.-zoot. Instit. W ii rzuurfl , Vol. n. 1875. 



(53) C. Semper. "Das Urogeuitalsystem d. Plagiostomeu, etc." Arbeit, a. d. 

 zool.-zoot. Iiiftit. Wiirzbtirfi, Vol. n. 1875. 



(54) Wyman. "Observations on the Development of Raja batis." Memoirs of 

 the American Academy of Arts and Sciences, Vol. ix. 1864. 



