THE INFANCY OF FISHES 22t 



packed within the walls of the ovaries, which are provided 

 with several folds of membrane, richly furnished with blood- 

 vessels and extending among the developing embryos, sup- 

 plying them at once with oxygen for respiration and with 

 nourishment. At first this nourishment is absorbed by 

 the general surface of the embryo. Later, when the 

 intestine is developed, the nourishing fluid is taken up 

 by it and digested, finding its way thither, not by the 

 mouth, which has not yet come into being, but through 

 the gill-slits, into which it is drawn by the vibratile 

 motions of certain delicate thread-like growths clothing 

 the walls of the gullet, which is pierced by these slits. 

 But the most surprising thing of all takes place when, 

 to ensure more efficient oxygenation of the blood, or in 

 other words more effective breathing, the fins become 

 pressed into service, and take up the work which later 

 will be performed by the gills ! For this purpose the 

 delicate skin which lies between the bony fin supports, 

 or rays, is produced beyond the edge of the fin, and supports 

 a delicate network of blood-vessels which absorb the life- 

 sustaining oxygen from the walls of the ovary. This is 

 really a most remarkable fact, and one without parallel 

 among fishes. 



Quite as extraordinary is the case of some of the skates 

 of tropical seas. The most famous is that of the Bat Ray 

 (Pteroflatea micrurd). During the early stages of its 

 growth it is sharklike in form, and provided with a yolk- 

 sac, like a young shark. The contents of this bag of 

 provender pass, in some way not understood, up the 

 hollow stalk by which it depends from the body, into the 

 intestine ; but a certain amount of yolk is also absorbed 

 by a veritable cloud of thread-like external gills, which 

 turn downwards and backwards to envelop the yolk-sac. 



