THE WONDER OF LIFE 



527 



fifteen and a half feet long, and found twenty-three em- 

 bryos in the oviducts. As each of these was about fourteen 

 inches long, including the toothed saw of five inches, one 

 naturally becomes curious as to the relation of the weapon 

 to the wall of the oviduct. Mr. Southwell found that the 

 teeth of the saw were ' entirely 

 covered by a transparent carti- 

 laginous tissue, which of neces- 

 sity must disappear later.' 



Every one who hves on the 

 coast is famihar with the egg- 

 cases of skate and dogfish, the 

 so - called mermaid's purses. 

 These are quadrangular sacs 

 with a long tendril at each of 

 the corners ; they are made of 

 jets or fluid filaments of keratin 

 which are secreted by a gland in 

 the oviduct and coalesce into a 

 flexible egg-case. There are no 

 hving cells in the egg-case itself ; 

 it encloses the large egg-cell laden 

 with yolk and floating in albu- 

 men or white of egg. When the 

 egg is liberated from the mother- 

 fish, the tendrils writhe automa- 

 tically in the water and twine 



round sea-weed on the floor of the sea in the shallow-water 

 area. Thus the eggs are saved from being smothered in 

 the drifting mud, and the developing embryos within are 

 gently rocked, and thus the better aerated, by movements 

 in the water. But how is the embryo to escape from its 



Fig. 83. — Mermaid's Purse, 

 or homy egg-case of 

 dogfish, with 

 ing tendrils. 



attach- 



