418 Mr. A. Alcock on 



mobrancli fishes in whicli tlie female develop3 during 

 pregnancy a vast system of uterine glands that secrete a 

 nutrient fluid, or uterine milk, for the nurture of the deve- 

 loping embryo. 



Jn this paper there will be given a detailed account of the 

 phenomenon as lately reinvestigated in the species — Tryrjon 

 Bheheri^ Blyth — in which it was first noticed by us. 



As is well known, reproduction among the Elasmobranchii 

 is effected by the internal impregnation of the female. 



In some, as in the familiar instance of the ray, the female 

 after impregnation lays eggs, which are enveloped in a tough 

 leathery capsule secreted by the oviduct. 



In others, as familiarly exemplified by many sharks, the 

 c^^ undergoes its changes and the embryo completes its 

 development in the terminal part of the oviduct, which is 

 now enlarged and elaborated to form a true uterus for the 

 reception and retention of the embryo. In this case, as has 

 long been known, a true placenta is formed, differing from 

 the Mammalian placenta in the particulars which follow from 

 the one main general fact that it is the yolk-sac, instead of 

 an allantois, that furnishes the foetal part of the structure. 



There is yet a second method of viviparity known to occur 

 among the Elasmobranchs, and to some particulars of it this 

 paper is devoted. In this method while, on the one hand, 

 the eg^ is retained and the embryo nourished within an 

 oviductal enlargement or uterus, on the other hand no sort of 

 vascular connexion is formed between the parent and the 

 foetus. Here the expenditure of tissue comes altogether from 

 the maternal side, the whole of the Q.^g being devoted to the 

 foetus and none of it being set aside to form vascular absorbent 

 structures. 



In passing, one cannot but remark upon the interesting fact 

 that in the primitive Elasmobranch group we find in co-exist- 

 ence all the methods of reproduction that occur in the higher 

 Vertebrate phyla, namely (1) oviparity, with large-yolked eggs 

 enclosed in a more or less rigid shell, (2) viviparity, with 

 the formation of a placenta, and (3) aplaccntal viviparity. 



So far as we are at present aware the method of utero- 

 gestation now under consideration reaches its perfection in 

 the Batoidei ; and of the six families into which this suborder 

 is divided it has been observed in three, namely the Torpe- 

 dinidfu, the Trygonidjp, and the ^lyliobatidiv. 



in Torpedo^ as Professor Wood-Mason and I have elsewhere 

 recalled, it was investigated in furthest detail by Dr. John 

 Davy, who, in pregnant females,* noticed (1) foetuses lying 

 naked in the uterus and unattached to it by any form of 



