FITNESSES 



with a long tendril at each of the corners ; they are 

 made of jets or fluid threads of horn which are 

 secreted by a gland in the oviduct and coalesce into 

 a flexible egg -case. There are no living cells in the 

 egg-case itself ; it encloses 

 the large egg-cell laden 

 with yolk and floating in 

 albumen or white of egg. 

 When the egg is liberated 

 from the mother-fish, the 

 tendrils writhe automati- 

 cally in the water and 

 twine round seaweed 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 with- 

 in are gently rocked, and 

 thus the better aerated, 

 by movements in the 

 water. But how is the 

 embryo to escape from its 

 closed cradle ? It ap- 

 pears that at the time 

 of hatching there is a 

 secretion from the embryo which acts as a sol- 

 vent on a weak seam at one end of the mer- 

 maid's purse. The end gapes, and the miniature 

 skate or dogfish works its way out. Now it is 

 interesting to find a parallel fitness in the far- 



63 



Mermaid's Purse, or horny 

 egg-case of dogfish, with 

 attaching tendrils. 



