34 -^ Guide to the Zoological Collections 



embryo developes in the mother's oviduct, and when all 

 the yolk is used up by the growing embryo, the large 

 empty yolk-sack adheres to the wall of the oviduct so 

 that the blood-vessels of the two structures — yolk-sack 

 and oviduct — come into close contact and allow nourish- 

 ment to pass from the mother to the embryo. 



In the same Case (12) a specimen of the unborn 

 young of a Sting-ray (Trygon) is shown lying in the 

 mother's oviduct. Here too the embryo developes 

 within the oviduct, of which the lining membrane is 

 shaggy with glands that secrete a kind of milk. When 

 all the yolk is used up by the embryo, the yolk-sack, 

 which is small, shrivels, and then the embryo lives on 

 the milk which passes into its mouth through the 

 spiracles. In the exhibited specimen clusters of milk- 

 glands are seen to pass deep down into the spiracles. 



There are certain Bony Fishes (see Saccogaster, Dip- 

 lacanthopoma , and Hephthocard) in which impregnation is 

 internal and the young do not leave the mother until 

 their development is complete. Here the eggs are 

 small, as in all Bony Fishes, and the growing embryos 

 appear to be nourished by simple absorption from the 

 wall of the ovary. 



In some fishes the sexes differ considerably in appear- 

 ance, and it is generally the male that differs from the 

 female, either in being more brilliantly coloured or in 

 possessing special ornaments. Numerous instances of 

 these " secondary " sexual differences have been 

 brought together by Darwin, in the Descent of Man. 

 In Case 13 two good examples are shown; in one of 

 them {Brachypleura) the male has the first few rays of 

 the dorsal fin greatly lengthened to form an erectile 

 plume : in the other (Callionymus) the male alone has 

 both the dorsal and the caudal fin-rays much pro- 

 longed. 



