424 CASWELL GRAVE 



all echinoderms, is represented by a specific part of the ground 

 substance predifferentiated in a definite region of the egg; 

 second, that these different substances, which ordinarily flow 

 together after the maturation divisions to form the lower stratum 

 of the middle zone of 'formative stuffs' and hence are carried 

 in together during gastrulation, fail for some reason to become 

 thus localized in the yolky eggs of Ophiura, Solaster, Cribrella, 

 Antedon, etc., and hence are not all carried forward together 

 during gastrulation; third, that the failure of these specific 

 substances to be united into one continuous mass is due to an 

 interference with the free action of the mechanical processes 

 which nonnally bring about localization, by the inertia of the 

 dense mass of yolk material with which the cytoplasm is filled; 

 fourth, that the eggs begin to segment before the retarded locali- 

 zation processes have been completed and thus prevent their 

 further action. 



Localization, however, is not altogether prevented, for in 

 Ophiura some of the mesodermal material (anterior pair of 

 enterocoeles) becomes morphologically differentiated at the 

 same place as in other echinoderms, but that part which forms 

 the remaining coelomic anlagen lags behind, some of it occupy- 

 ing a place in the midst of the endodermal substance while 

 some of it fails even to get into the middle zone, but remains 

 in the midst of the upper polar zone of ectodermal substance. 



By a comparative study of the development of the eggs of 

 those species which, in the places of origin of the essential coelo- 

 mic structures, differ from Arbacia or Strongylocentrotus, it 

 should be possible to throw considerable light upon the funda- 

 mental organization of the echinoderm egg, for, by noting the 

 various points at which each of the several mesodermal struc- 

 tures makes its first morphological appearance and the relation 

 it bears to other larval structures, the approximate location 

 within the undeveloped egg of the original material out of which 

 each takes its origin can be established. The amount of ma- 

 terial available for such a comparative study is large. More 

 or less divergence from the normal sequence in the origin of 

 mesodermal structures can be anticipated in the development 



