434 Wyman's Observalions 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 fœtus 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 toa placenta. 
In Spinax, the vessels on the surface of the vitelline sac - 
brought in close contact with the highly vascular folds which 
line the oviduets. But in Carcharias, as Müller has demon- 
strated in his Memoir on the subject, not only is there an ap- 
proximation of the fetal 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 opposins 
surfaces become adherent even. In both Spinax and "- 
charias, the necessary conditions exist for the reaction of ma- 
ternal and fcetal blood upon each other, as is the case im the 
Mammalia, but to a much more limited extent.* 
* Dr. John Davy has shown, that in Torpedo the embryo is nourished at the 
se of ials furni 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. s 
