438 CASWELL GRAVE 



it necessarily enters to make the Echinid-Ophiurid correlation 

 curve less accurate than is to be desired. 



The size (diameter) of egg, which is one of the factors used 

 in constructing the curve, is again not the real factor involved 

 in the correlation. The real factor, the ratio between the vol- 

 umes of living substance and yolk substance of the eggs, which 

 it is not possible to measure or estimate, is most nearly approxi- 

 mated by a comparison of diameters of eggs, assuming that the 



Text fig. 3 The relative sizes of the eggs of certain echinodernis are here 

 shown by circles. 



C, the egg of Ophiocoma (of minimum size), superimposed upon D, the egg 

 of Ophiura (of possible maximvmi size), to show the relative difference in size 

 of the eggs of Opliiurids due to difference in yolk content. 



A, the egg of Toxopneustes, and B, the egg of Mellita, brought together to 

 show the same for the eggs of Echinids. 



E, the egg of Toxopneustes, and F, the egg of Asterias, superimposed to show 

 the great difference in size between eggs of the minimum size characteristic of 

 Echinids and Asterids. 



volume of living substance in all eggs of the Echinid-Ophiurid 

 series is substantially the same. 



Apparently a reduction of the amount of yolk material in 

 an egg results in prolonging the period of its development, but 

 there is a minimum limit to which such reduction may be car- 

 ried, namely; to an amount insufficient for the establishment 

 of those larval structures and fimctions necessary for the cap- 

 ture and digestion of plankton organisms. In the minute eggs 



