434 Wyman's Observations on 



a transition to the next division, where the conditions of de- 

 velopment are wholly changed. 



Extended observations have proved, that a large number of 

 species of fishes, belonging to many genera, are truly vivi- 

 parous, the foetus passing through a real gestation by the 

 parent before its development is complete. These Vivipa- 

 rous fishes may be divided into two groups, according to the 

 position occupied by the embryo during the period of its 

 growth. 



I. In the first group may be arranged those fishes in which 

 the ovum leaves the ovary in an undeveloped state, and in 

 which the process of eolution is not commenced until it 

 reaches the lower portion of the oviduct. The species which 

 this group comprises are nearly all, if not all, Plagiostomes. 

 The best known are Spinax, Carcharias, Mustellus, Galeus, 

 and Torpedo. Although they are usually classified among the 

 lowest of fishes, it is in some of them that the process of repro- 

 duction becomes most nearly analogous to that of the highest 

 Vertebrates. Not only does the yelk reach proportions like 

 those of the yelk of birds, but the yelk-sac itself plays the part 

 of an allantois, and forms an organ analogous to a placenta. 

 In Spinax, the vessels on the surface of the vitelline sac are 

 brought in close contact with the highly vascular folds which 

 line the oviducts. But in Carcharias, as Muller has demon- 

 strated in his Memoir on the subject, not only is there an ap- 

 proximation of the foetal and maternal vessels, but the sur- 

 faces of the yelk-sac and of the oviduct are both deeply convo- 

 luted, 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 ma- 

 ternal and foetal blood upon each other, as is the case in the 

 Mammalia, but to a much more limited extent.* 



=* Dr. John Davy has shown, 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 egg at the time development commenced. 

 Philos. Trans. 1834. On the development of the Torpedo. 



