Evidence -from Somatic Histogenesis 



45 



confirm Huntsman's results. As Huntsman had opportun- 

 ity to study the matter in a larger number of species than 

 Miss Forsyth and I have had, the following description is 

 taken largely from his paper. Figures 44a, b, 45, 46 are 

 from Huntsman and figure 47 is from Ritter and Forsyth. 

 By comparing figures of what may be called the dorsal view 

 (figures 44a, 45, 46) with the side view (figure 44b) it is 

 seen that the distinctive feature about the cell which con- 

 stitutes the spinule is a shield-like plate on one side of the 

 somewhat elongated cell, the distal end of which projects 

 more or less beyond the cell body, the whole resembling to 



FIGURE 45. SPINULE CELL OF STYELA PLICATA (AFTER HUNTSMAN). 



some extent the end of a finger with its nail. The shield 

 is harder than the rest of the cell, and is probably com- 

 posed of the same material as the general "test" of Ascid- 

 ians, animal cellulose. The spinules are so placed that the 

 basal portion is embedded in the surface layer of the test 

 on the inside of the siphon, the shield being on the free side 

 of the cell with its free edge pointed toward the lumen of 

 the siphon. The specific attributes furnished by the 

 spinules depend upon the shape and structure of the shields. 

 The free edges may be truncate (figure 44, S. yakuta- 

 tensis), or long-pointed (figure 46, S. greeleyi), or low- 

 conical (figures 45, 47, S. plicata, S. montereyensis) . 

 Again the edge may be smooth (figures 46, 47, 5. greeleyi 



