Experimental Studies on Germinal Localization. 15 



in the meantime divides equally, without the formation of a polar 

 lobe. In the 4-eell stage, accordingly, the large posterior cell, 

 D, exceeds A, B or C, by exactly the volume of the lobe, and 

 the lower white area appears only in D (Fig. 14). On the other 

 hand, the substance of the original upper white area is equally 

 distributed among the four; but it is evident that the amount of 

 white material visible at the surface has somewhat increased. 

 The 4-cell stage shows the characteristic relations of the blasto- 

 meres observed in so many other eggs of this type. The two 

 lateral cells, A and C, He at a higher plane, and are In contact 

 along the upper side by an upper "cross-furrow." B and D, on 

 the other hand, are in contact along a longer transverse lower 

 cross-furrow; and these characters, together with the large size 

 of the posterior cell, D, thus give an immediate means of orienta- 

 tion from this time forwards. 



As the egg prepares for the third cleavage the upper white 

 material shifts slightly towards the left upper angle in each quad- 

 rant, anticipating the formation of the first quartet of ectomeres 

 by the usual dexlotropic cleavage. These cells, which are of 

 equal size and In the A, B and C quadrants are not much smaller 

 than the basals, are formed entirely from the white material of 

 the upper polar areas; and it Is here again evident that an ex- 

 tensive flow of this material must take place from the interior 

 of the egg. Their formation does not, however, exhaust the 

 white substance of the upper areas, which still remain In the 

 upper regions of the four basals. During this division the polar 

 lobe forms for the third and last time, from the white material 

 of the lower area, in the D-quadrant; but it Is now noticeably 

 smaller than before, and does not constrict so deeply (Fig. 15). 

 After the completion of the cleavage the lobe again fuses with 

 D, in which, as the egg enters Into the "resting stage," the lower 

 white area still appears; though this soon undergoes a great 

 change (Fig. 16). 



The fourth cleavage is of especial interest, since a large part 

 of the substance of the lower white area now passes into the first 

 somatoblast, id, or X, and Is thus for the first time actually cut 

 off from the pigmented region. This cleavage is preceded and 



