OPHIURA BREVISPINA 431 



development, similar in effect to that observed in Ophiura, we 

 find them in abundance among the Mollusca, Arthropoda and 

 Vertebrata. In each of these phyla the same result is accom- 

 plished in a way different from that which has been developed 

 in the others, but within each group the method of yolk segre- 

 gation is essentially the same. In species of molluscs the yolk 

 material is chiefly locahzed within one portion of the egg and 

 becomes included during segmentation within a comparatively 

 few inert cells. Among arthropods and vertebrates the living 

 substance of the egg withdraws more or less completely from 

 the yolk material and undergoes development and differentia- 

 tion practically independent of it. In all of these cases the 

 general effect of the segregation of the yolk substance is essen- 

 tially the same; the energy of the developing organism is there- 

 after not expended in a constant manipulation of yolk, but is 

 available wholly for activities that result in development and 

 differentiation. 



The processes of yolk segregation as they are found in numer- 

 ous species of various genera of Gasteropods are so nearly iden- 

 tical in every way, including the particular cells involved, that 

 it is difficult to conceive of them as not as much a part of the 

 common inheritance of the group as are its common structural 

 characters. The same may be said of the modes of yolk segre- 

 gation pecuhar to insects, reptiles and birds, but in the cases 

 of Ophiura and other widely separated species of echinoderms 

 the processes seem to have been developed so abruptly, are so 

 isolated and different as to lead to the conclusion that they 

 have been independently acquired and are not genetically con- 

 nected. A specific process or mechanism of yolk segregation, 

 needing only the stimuli of a changed internal environment 

 (increase of yolk) to bring it into function, does not seem to 

 be inherent in the echinoderm egg, because apparently the same 

 internal environment which brings into function a process of 

 segregation of one type in Ophiura, results in segregation of 

 different types in the yolky eggs of other echinoderms. 



Cerianthus, Actinia, Urticina. The Coelenterate phylum was 

 not included in the foregoing list of those in which characteristic 



