1885.] PttOCEEDIlS^GS OF UNITED STATES NATIONAL MUSEUM.' 129 



Ji^-iir: . 



Tal.TllI, ]Vo. 9. \¥ashing:ton, II. €. msty 25, 1883. 



ovum leaves the ovary in an undeveloped state, and in which the process of 

 evolution is 7iot commenced until it reaches the lower portion of the oviduct. 

 The species which this group comjnises are nearly all, if not all, Pla- 

 giostomes. The best known are Spinax, Carcharias, 31ustelus, Galeus, 

 and Torpedo. Although they are usually classified among the lowest 

 of fishes, it is in some of them that the process of reproducti»« be- 

 comes most nearly analogous to that of the highest Vertebrates. Kot 

 only does the yelk reach proportions like the yelk of birds, but the 

 yelk-sac itself plays the part of an allantois and forms an organ analo- 

 gous to a placenta. In Spinax. the vessels on the surface of the vitel- 

 line sac are brought into close contact with the highly-vascular folds 

 which line the oviducts. But in Carcharias, as Miiller has demonstrated 

 in his memoir on the subject, not only is there an approximation of the 

 foetal and maternal vessels, but the surfaces of the yelk-sac and of the 

 oviduct are both deeply convoluted, and the projections of the one are 

 admitted into and embraced by the concavities of the other, and the 

 opposing surfaces become adherent even. In both Spinax and Car- 

 charias the necessary conditions exist for the reaction of maternal 

 and foetal blood upon each other, as in the case in the mammalia, but 

 to a much more limited extent.* 



"II. In the second group those fishes are comprised in which the ges- 

 tation is either wholly or in part ovarian, the last stages only of the i3ro- 

 cess usually occurring in the oviduct. Among the genera included in 

 this division are Silurus,\ Blennius,\Anahleps,\ Pcecilia^W ixwAEmhiotoca.^ 

 In all of these genera impregnation takes place in the ovary, and, as 

 seems probable, while the ovum is still invested with its original envel- 

 opes. In Bleuny, Eathke has shown, the ovarian gestation having con- 

 tinued about three weeks, that about the end of September the sac rup- 

 tures, and that the embryo is discharged into the central cavity of the 

 ovary, which is in fact the oviduct ; here the foetus remains till the be- 

 ginning of January, when it is born. In Pcecilia the foetus is liberated 

 and escapes into the oviduct towards the end of gestation. Valenciennes 

 has given several details in relation to the development of A na&ieps 

 Gronovii, made for the most part upon specimens in an advanced stage 

 of foetation, the smallest embryo being more, than an inch long. He 



* Dr. John Davy lias sliown that in Torpedo the embryo is nourished at the expense 

 of materials furnished by the parent, since the mature foetus weighs more than twice 

 as much as the egfr at the time development commenced. Philos. Trans., 1834. On 

 the Development of the Torjjedo. 



tCuvier et Valenciennes, Hist. Nat. des Poissons, t. i., p. 540, 1828. 



t Rathk6, Mem. sur la Develop, de I'Homme et des Animaux, 2™^ partie, Leipsic, 1833. 



^ Cuv. and Valenciennes, Hist. Nat. des Poissons, t. xviii, p. 245, Paris, 1846. 



IIDuvernoy, Ann. des Sc. Nat., t. i,3«, ser., p. 313, 1844. 



U Agassiz, Am. Journ. of Science, xvi, 2d ser., Nov., 1853. 

 Proc. Nat. Mus. 85 9 



