YOLK-SACS 137 



at the surface of the sea by means of a single 

 large oil-globule ; four or five days after oviposi- 

 tion the embryos are hatched ; four or five days 

 after hatching the yolk has become absorbed. 

 The buoyancy of the yolk-sac causes the larvae 

 to float helplessly for some time after hatching, 

 with the yolk-sac uppermost. 



The accumulation of yolk in eggs, or the 

 lecithality of the ovum, also offers numerous 

 examples of convergence as between cartilaginous 

 fishes, bony fishes, and sauropsida (birds and 

 reptiles) as well as amongst invertebrate animals. 

 The large yolk-sacs of Arius and of Gymnarchus 

 have no genetic relation to each other nor to 

 those of sharks and rays. The yolk - sac of 

 cephalopod molluscs is a structure sui generis^ 

 and it is particularly noteworthy that the most 

 ancient existing genus, Nautilus, has the most 

 macrolecithal egg of all whose eggs are known. 



