OPHIURA BREVISPINA 423 



phological differentiations which follow upon development 

 but are only the result of differentiations of the more funda- 

 mental underlyizig ground substance of the egg. The fact 

 remains therefore that there lies back of the 'formative stuffs' 

 of the unsegmented egg real differentiations of the ground sub- 

 stance of at least three kinds, and that they have, prior to mor- 

 phological development, a localization about the axis of the 

 egg the same as that shown by the visible formative stuffs. 



The developmental histories of the numerous echinoderm 

 larvae which have been studied indicate strongly that a promor- 

 phological differentiation of the ground substance and localiza- 

 tion of formative materials, such as that shown to exist in Ar- 

 bacia and Strongylocentrotus, holds true for and is characteristic 

 of the eggs of echinoderms in general. Embryological studies 

 indicate further that the middle zone of this predifferentiated 

 material is composed of at least two distinct kinds of substance, 

 the first being that which is destined to pass into mesodermal 

 (coelomic) structures, the second to endodermal organs. In 

 practically every case which has been followed, the coelom and 

 its derivatives are differentiated out of material localized in 

 a group of cells which, in the gastrula, occupies the distal or 

 free end of the archenteron. These materials separate from 

 the endodermal part of the archenteron in the form either of 

 a bilateral pouch or a pair of pouches at a time just before the 

 remaining proximal portion of the archenteron unites with the 

 ventral ectoderm to form the mouth. In practically all echino- 

 derm larvae this first enterocoelic pouch or pair of pouches 

 contains all of the material out of which, by a definite series 

 of constrictions or divisions, the entire series of mesodermal or 

 coelomic structures of the adult are differentiated. 



The great differences between Ophiura and other ophiurids 

 in the places of origin of evidently homologous structures, and 

 the anomalous organizations and peculiarities of the larvae of 

 Solaster, Antedon and all echinoderms producing large yolky 

 eggs (text fig. 2, C, E, F), may be satisfactorily harmonized 

 and accounted for if we make certain fully warranted assum- 

 tions: first, that each of the coelomic structures, common to 



