I20 



WHITMAN. 



[Vol. I. 



Possibly the order in Nephelis is variable, in which case 

 ButschU's observations on this point would appear supple- 

 mentary, rather than contradictory, to those of Robin. 



BUtschli failed to trace the cleavage in detail beyond the 

 sixteen-cell stage, in which (Fig. 5, PI. XVIII.), as I have 

 shown, every group of segments, and nearly every individual 

 segment even, can be identified with those of the correspond- 

 ing stage in Clepsine. The close correspondence in parts, 

 relative size, position, and axial orientation, is shown in Diags. 

 5 and 6. The bulk of the egg is formed of the three macro- 



Tzvo views of the sixteen-cell stage of Nephelis. Three deep cells, represetttijtg the 

 first entoderm cells, are not shozvn. 



DiAG. 5. — Seen as a transparent object from the upper side. (After 

 BUtschli.) 



DiAG. 6. — Side view of the same stage. (Constructed after BUtschli and 

 Robin.) 



a, left macromere ; c, median and anterior macromere ; b, right macromere ; 



aa, cc, XX, bb, micromeres derived from a, c, b, and x. 



x'x' and x'x' micromeres derived from the neuro-nephroblast («')> "which is 

 now represented by two cells, x' and x'. 



xy, left mesoblast; x, right mesoblast. 



meres, one (r) of which has an anterior median position, while 

 the second and third (^a and F) are laterally placed. The six 

 micromeres are arranged in two bilaterally symmetrical rows, 

 extending from the upper angle of the anterior macromere to 

 the hind edge of the ^^-g. The two mesoblasts (^x and xy^ are 



