No. 447] HA MINE A SOLI T ARIA SAY. 



219 



The living egg of H. solitaria is so small and so richly sup- 

 plied with deutoplasm that satisfactory observations on the 

 segmentation are impossible except in the early stages. The 

 egg is spherical, enclosed within a thin structureless membrane. 

 The size of the egg varies, the average is about .08 mm., 

 being smaller than the eggs of Umbrella (Heymons, 93), 

 Crepidula (Conklin, '97), Nucula (Drew, :oi) and that of 

 most molluscs that have been studied. 



Before segmentation the polar differentiation of the egg is but 

 slightly indicated, the yolk being almost uniformly distributed 

 except in the region of the polar bodies. It has already been 



stated that it takes forty minutes for the animal to lay a mass 

 of eggs. Within ten or fifteen minutes after each egg is laid 

 the first polar body appears at the animal pole and thirty 

 minutes later the second i)olar body can be seen. It happens 

 occasionally that the first polar body is very large and may 

 even contain yolk spheres. 



The egg segments into two cells a half hour after the second 

 polar body has appeared. In about thirty per cent, of the eggs 

 observed, the first division of the egg did not divide it into two 

 equal blastomeres, one being noticeably larger, a variation which 



