Experimental Studies on Germinal Localization. 233 



The CD half, which clearly shows both upper and lower white 

 polar areas (Fig. 88), forms a polar lobe from the lower white 

 area and passes through a trefoil stage, nearly similar to that of 

 a whole egg (Fig. 89). Measurements show, however, that the 

 polar lobe is always proportionately larger than in a normal tre- 

 foil, being often as large as in a whole egg, though sometimes 

 more or less reduced. The lobe subsequently fuses with the pos- 

 terior cell, D, producing a 4/2-cell stage closely similar to a nor- 

 mal 2-cell stage, except that the inequality is greater (Fig. 90). 

 At the second cleavage the polar lobe forms again (Fig. 91) 

 from the larger cell, D, which divides unequally and dexiotropl- 

 cally to form id, while the smaller cell, C, divides slightly un- 

 equally to form ic. As in the whole egg the polar lobe then 

 fuses with D, producing an 8/2-cell stage that is essentially like 

 the posterior half of a normal 8-cell stage (Fig. 92). The fol- 

 lowing cleavage is especially interesting, corresponding again 

 with the divisions in the posterior half of a whole egg (Fig. 93). 

 All the divisions are leiotropic. The two upper cells divide 

 slightly unequally to form the two primary trochoblasts ( ic^, id^) 

 and the slightly larger upper cells (ic^ and id^). From iC 

 arises the right cell (2c) of the second quartet, while from iD 

 arises the first somatoblast (2d), which Is as large as in a whole 

 embryo, and in like manner is mainly formed from the lower 

 white area in \D. The 16/2-cell stage has, therefore, exactly 

 the same origin and composition as the posterior half of a whole 

 egg, consisting of six white ectomeres (ic\ id\ ic", id", 2c and 

 2d), of which 2d is the largest, and of two macromeres (2C, 

 2D), which contain all of the pigment and show each an upper 

 white area (Fig. 93). 



The history of isolated 54-blastomeres is entirely analogous. 

 The A, B or C quadrant typically divides slightly unequally, with- 

 out a polar lobe (Fig. 94), the smaller cell being composed of 

 white material and the pigment remaining in the larger; but cases 

 are not infrequent In which the division is nearly or quite equal. 

 The D-quadrant, on the other hand, forms a polar lobe, which, 

 as in a whole embryo, is typically much smaller than either the 

 first or the second (Figs. 95-96). The 2-cell stage is very un- 



