OPHIURA BREVISPINA 435 



echiiioderms of all classes are considered, the egg of the star- 

 fish Cribrella oculata (about 1 mm. in diameter) is probably 

 as large as is known, and the egg of the sea-urchin, Toxopneustes 

 variagatus, (about 0.065 mm. in diameter) probably stands 

 near the other end of the echinoderm series. 



Size of egg may depend upon more than one factor, one egg 

 owing its superior size to a relatively greater amount of living 

 or ground substance than another, but, with an exception to 

 be noted later, the differences in the size of the eggs of echiiio- 

 derms are due almost exclusively to differences in yolk con- 

 tent and not to differences in volume of ground substance. 



Conklin ('07) finds an even greater diversity in the sizes of 

 the eggs of gasteropods than is found among those of echino- 

 derms, the egg of Fulgur be>ing two thousand times the volume 

 of the egg of Crepidula plana, and he has observed that the 

 difference in the sizes of the eggs of gasteropods is also due 

 almost exclusively to increased quantity of the yolk. 



After a comparative study of the influence of increased volume 

 of yolk substance upon development in gasteropods, Conklin 

 concludes that it has produced little effect upon the organiza- 

 tion of the egg or upon cleavage, but that it profoundly modi- 

 fies the processes of gastrulation and organ formation. 



In Fulgur and numerous other gasteropods, any effect the 

 increased volume of yolk might have upon the rate and duration 

 of larval development is neutralized by the protection which 

 is afforded by the parent to the eggs and embryos during the 

 entire developmental period, the young gasteropods escaping 

 from the egg capsules in the adult condition. The cause for 

 such a correlation is obscure, but that the correlation is real 

 seems to be established by many observations. 



The highly specialized larval forms of echinoderms (bipin- 

 naria, auricularia, echinoplutei and ophioplutei) , having com- 

 paratively long periods of larval existence, terminated by a more 

 or less abrupt metamorphosis, develop from eggs in which the 

 supply of yolk substance is relatively small. On the other hand 

 rapidly developing larvae, simple in external form and without 



JOURXAL OF MORPHOLOGY, VOL. 27, NO. 2. 



