146 MARY J. GUTHRIE AND HOPE HIBRARD. 



after gastrulation, to form large numbers of mesenchyme cells 

 while still in the blastula stage. 



In contrast with these processes, Masterman (1902) in Cribrella 

 ocidata, and Gemmill (1912) in Sol-aster endeca,have described a 

 very different series of events. In Solaster the cleavage results 

 in a solid morula of nearly equal cells. The outside of this 

 mass then becomes wrinkled by the appearance of convoluted 

 furrows which so increase the surface that the cells inside can 

 become arranged in a single layer, leaving a flattened, irregular 

 cavity, the blastoccele. The furrows gradually disappear as the 

 cells over the entire surface become longer and narrower until 

 the larva becomes smooth once more. The initiation of the 

 process of gastrulation also aids in flattening the surface. During 

 these changes some wandering cells fail to arrange themselves 

 in a layer with the others and are left behind in the cavity of the 

 blastula. These are the primary mesenchyme cells. In Cribrella 

 similar phenomena occur, but instead of being left behind as the 

 morula changes to a blastula, the mesenchyme cells are budded 

 off after invagination, from the "hypoblast" into the blastoccele, 

 in the manner of the more typical Asteroidea. In addition to 

 this, cells morphologically similar to mesenchyme cells are budded 

 off from the other side of the "hypoblast" into the archenteron, 

 and were called "hypenchyme" by Masterman. 



Even fewer species of the Ophiuroidea have been investigated 

 on this point. Grave (1916) in Ophinra brevispina found a large 

 number of mesenchyme cells in the blastocoele of early stages. 

 MacBride (1897) observed mesenchyme in blastula? of Ophiothrix 

 fragilis between four and five hours after fertilization, while 

 gastrulation did not begin until about the eighteenth hour. 



The majority of Echinoidea show early mesenchyme formation. 

 MacBride (1903) figured blastula? of Echinus miliaris, E. acut-us 

 and E. esculentus with mesenchyme forming at one end, which 

 was, jn some cases, slightly flattened. In E. esculentus he found 

 about fifty mesenchyme cells which were probably derived from 

 the micromeres in the blastula. In Echinocardium cordatum 

 (MacBride 1914) mesenchyme is found at the vegetal pole long 

 before any signs of gastrulation. MacBride was unable to con- 

 linn the observations of Flcischman 'ixxxi on this form with 



