Experimental Studies on Germinal Localization. 17 



The fifth cleavage, dexlotropic in all the cells, produces the third 

 quartet, each cell of which is considerably smaller than the corre- 

 sponding basal (Fig. 19). Qf these cells 3d is much the largest, 

 and is usually composed entirely of white material, while 3a, 3b 

 and 3c usually, perhaps always, receive a certain amount of 

 pigment. At the end of the cleavage the macromeres rapidly 

 diminish in apparent size, evidently owing to their passing more 

 deeply into the egg, and the color-pattern becomes more or less 

 confused, though A, B and C still show the greatest amount of 

 pigment, while D distinctly shows a white area on the side turned 

 towards X, where 4d is subsequently formed. I have not been 

 able to observe the formation of the entire fourth quartet satis- 

 factorily, either in the opaque living object or in preparations. 

 I can however state positively that as seen in surface-view of the 

 living egg, 4d is very small (smaller than 3d and very much 

 smaller than 2d) and appears pure white (Fig 19, a). I have 

 been unable to determine whether the white material of this cell 

 is derived from that of the original lower white area; though, 

 as will appear hereafter, the experimental evidence indicates that 

 such is the case. At this period the four basals appear much 

 smaller, having evidently retreated into the interior. 



Beyond this point it is not necessary at this time to trace the 

 cleavage. The foregoing observations clearly show that, in Den- 

 talium the freshly discharged egg, prior to maturation or fer- 

 tilization, shows a definite segregation of zisibly different ma- 

 terials which accurately foreshadows a corresponding distribution 

 of these materials among the hlastomcres during cleavage. Of 

 the three zones of material superficially visible in the living egg, 

 the upper one (upper white area) is allotted to the first three 

 quartets of ectomeres, apparently in equal amount in each quad- 

 rant; the middle pigmented zone is mainly allotted to the four 

 basal entomeres, though a portion also passes into ectomeres of 

 the second and third quartets; while the lower zone (lower white 

 area) certainly passes mainly into the first somatoblast, 2d, or 

 X, probably in part into the second somatoblast, 4d, or M, 

 and possibly in part into the left posterior micromere, 3d, of the 

 third quartet. This agrees in general with the history of the 



