OPHIURA BREVISPINA 427 



tribution and form of the yolk material of the organism has 

 been effected. 



By reference to figures 5 and 6 it will be seen that by far the 

 greater number of the yolk spherules orginally contained within 

 the cells have been extruded and are now to be found lying free 

 in the segmentation cavity.- Each cell, however, has retained 

 in its cytoplasm a small amount of its original supply of yolk. 

 The yolk material that has been extruded from the cells is ap- 

 parently the greater part of that which formerly made up the 

 numerous group of spherules located in the parts of the cells 

 situated between the nucleus and the segmentation cavity, 

 for now this group in each cell consists of a comparatively few 

 spherules. Apparently all of the yolk spherules which com- 

 posed the group situated between the nucleus and the base of 

 the cell have been retained within the cells. 



These figures are reproductions of drawings of cross sections 

 of a larva in the stage of development in which the coelom and 

 its derivatives are first differentiated, such as is shown in figure 

 4. A further study of the sections shows that the yolk spherules, 

 so long as they remain within the cells and are surrounded by 

 cytoplasm, are relatively small particles and quite uniform in 

 size, but that they fuse together to form much larger masses 

 of varying size and shape when extruded into the segmentation 

 cavity where they are practically free from cytoplasm. 



Among the free yolk masses in the segmentation cavity mesen- 

 chyme cells may be seen here and there. Possibly the function 

 of these cells is that of ingesting the segregated yolk material 

 and redistributing it to parts of the larva where energy is most 

 rapidly expended. 



The result of the yolk segregation process in Ophiura is that 

 a more or less complete separation of the active living sub- 

 stance of the developing individual from the inert non-living 

 yolk material is effected, enabling the living substance there- 

 after to continue its development practically independent of 

 and unhindered by the yolk; independent to this extent at least, 

 that the individual cells are no longer under the necessity of 

 expending energy in a constant manipulation of 3' oik. 



