432 CASWELL GRAVE 



processes of yolk distribution take place during or preceding 

 development for the reason that the process as commonly found 

 among coelenterates is somewhat obscured by other develop- 

 mental phenomena and has therefore been subject to widely 

 different interpretations. It is in this phylum, however, that 

 cases of yolk segregation have been described which seem so 

 nearly parallel to that which takes place in Ophiura as to merit 

 careful comparison. 



Kowalevsky (73), Jourdan (79-'80) and Appellof ('00) 

 have described and figured stages in the early development of 

 Cerianthus membranaceus, Actinia equina and Urticina crassu- 

 laria, respectively, in which the originally empty blastocoele 

 or archenteron, or both, later become filled with free yolk ma- 

 terial. Kowalevsky found in Cerianthus membranaceus that, 

 after a typical invaginate gastrula has been formed from a 

 one layered hollow blastula, the entoderm 'secretes' a mass 

 of free yolk substance into the archenteron. Jourdan describes 

 an invaginate gastrula also for Actinia equina and states that 

 the free yolk which appears in both bla^ocoele and archenteron 

 seems to 'exude' from the larval tissues, but Appellof, after study- 

 ing the development of this species, failed to find an invaginate 

 gastrula, and, after critically examining Jourdan's figures, 

 concluded that Jourdan was mistaken, in part at least, in the 

 interpretation of his observations. Both investigators agree, 

 however, that in this species a once hollow blastula later becomes 

 filled with free yolk material. Appellof has followed the process 

 of yolk transfer as it occurs in Urticina in considerable detail, 

 and as this is the only case which has been studied carefully, 

 the following brief resume of his observations is given: In 

 Urticina crassicornis the unsegmented egg has a structure simi- 

 lar to that of typical coelenterates, a peripheral layer of clear 

 cytoplasm and a central portion filled with yolk material. Dur- 

 ing segmentation a blastocoele appears and from the first this 

 cavity contains a small quantity of granular material (central 

 yolk substance) which is not incorporated within the blasto- 

 meres. The completed hollow blastula consists of a single layer 

 of high columnar cells, each with its outer end made up of clear 



