OPHIURA BREVISPINA 443 



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SUiMMARY AND CONCLUSIONS 



1 . The great increase in the yolk content of the egg of Ophiura 

 brevispina, to which its large size is due, has not disturbed its 

 early developmental processes. The manner and rate of its 

 segmentation and the structure of the blastula remain practi- 

 cally the same as in the egg of Ophiocoma which is one seventy- 

 eighth the size of the egg of Ophiura. 



2. The coelomic structures of Ophiura take their morphologi- 

 cal origins at three widely separated parts of the larva which 

 might be taken to indicate that the organization of the egg of 

 Ophiura is fundamentally different from that of echinoderms like 

 Arbacia, Strongylocentrotus, etc., in which the mesodermal 

 material has been shown to be localized in the egg, at the time 

 of fertilization, in a middle horizontal zone and carried out 

 during gastrulation in one mass to the end of the archenteron, 

 there and then to be distributed as five coelomic structures. 

 Assuming, however, that the eggs of all echinoderms are essen- 

 tially ahke in fundamental structure and promorphological 

 differentiations, the peculiarities shown by Ophiura, and all 

 other species having large yolky eggs, in the manner and place 

 of appearance of their coelomic anlages, may be satisfactorily 

 explained on the ground that each mesodermal organ is to be 

 referred back in all echinoderms to a definite substance formed 

 and localized in a definite part of the egg, and that these sub- 

 stances, which ordinarily flow together when fertilization takes 

 place, are prevented from so fusing in large yolky eggs by the 

 mass of inert yolk material with which such eggs are densely 

 packed. 



3. The yolk material of the egg influences its later develop- 

 ment by mechanically interfering with and hindering intra- 

 and intercellular movements. These effects are clearly apparent 

 in the blastula of Ophiura. 



4. The early gastrula of Ophiura responds to the presence of 

 this large amount of yolk material, and its inhibitory effect upon 

 developmental processes, by extruding the greater part of the 

 yolk substance of each of its cells into the segmentation and arch- 



