38 Edmund B. Wilson. 



between the first and second cleavages the "apical stuff" (if such 

 a term be allowed) exerts once for all its specific effect. The 

 most natural explaaation of this is given by the hypothesis that 

 this stuff moves upward to the apical pole, to be isolated in the 

 large posterior quadrant, D, during the second cleavage, and 

 subsequently in the corresponding micromere, id, during the 

 third cleavage. The basis of correlation between post-trochal 

 region and apical organ may thus be sought in the physical as- 

 sociation of the corresponding specific stuffs in the first polar lobe, 

 while the specification of the posterior micromere, id, is due to 

 the final isolation within it of the "apical stuff." 



V. 



LOCALIZATION IN THE UNSEGMENTED EGG. 



The preceding sections are in a measure only preliminary to 

 the present one which includes the most important part of the 

 present paper, namely, the results of experiments on the localiza- 

 tion of the polar lobe, and of the structures that it involves, in 

 the unsegmented egg. As has already been stated, the clear sub- 

 stance forming the polar lobe is already visible in the egg prior 

 not only to cleavage, but even to fertilization and maturation. 

 Experiments on the unsegmented egg show with great clearness 

 that this area possesses in a general way the same promorpho- 

 logical value as the polar lobe itself, though at this early period 

 the egg possesses a greater regulative capacity than at later stages. 

 The unfertilized living eggs of Dentalium may readily be cut in 

 two with the scalpel under the microscope, and the plane of sec- 

 tion determined with considerable accuracy not only during the 

 operation but by a subsequent examination of the fragments in 

 which the polar areas are often still clearly visible. As Yves 

 Delage first showed, such fragments when fertilized may segment 

 and give rise to ciliated embryos and in certain cases even to 

 dwarf trochophores. In a considerable proportion of such ex- 

 periments, both fragments develop. For convenience of descrip- 

 tion I shall divide them into two classes, including (a) those 

 obtained by horizontal or oblique section, and (b) those obtained 



