Experimental Studies en Germinal Localization, 13 



area passes into the lobe (like the green material of the Myzos- 

 tovia egg) to constitute its main bulk, precisely as Wheeler shows 

 in Myzostoma {cf. his Fig 47). The term "yolk-lobe" em- 

 ployed by a number of earlier observers is therefore as mislead- 

 ing as it is inappropriate and may be replaced by the term "polar 

 lobe." For reasons given in the discussion at the end, I believe 

 it very probable that at least the lower protoplasmic area, and 

 probably also the upper disc, are in a general way comparable to, 

 if not identical with, the polar rings observed in the eggs of cer- 

 tain leeches and oligochaetes. 



Immediately after the polar lobe is formed a vertical furrow 

 cuts into the egg from the upper pole, dividing the upper white 

 area into equal parts and forming with the polar lobe a trefoil, 

 of which the two upper lobes are of exactly equal size and contain 

 all of the pigment, while the unpigmented polar lobe is consider- 

 ably less than half the bulk of each of the others (measurements 

 give a ratio of i to 0.32-0.46, Fig. 6) . At the height of its form- 

 ation the trefoil appears at first sight to consist of three separate 

 spheres. Close examination invariably shows however that the po- 

 lar lobe Is united to one of the upper lobes by a very narrow pedicle 

 which is never severed; and as the cleavage proceeds these two 

 lobes completely fuse while the remaining upper lobe Is cut off as 

 a separate blastomere. Thus Is formed a characteristic unequal 

 2-cell stage (Fig. 7), consisting of a smaller anterior cell, AB, 

 and a larger posterior one, CD, which differ In volume by ex- 

 actly the bulk of the polar lobe. Each of these cells has at the 

 upper pole a white area, representing half the original upper 

 polar area. The lower polar area, on the other hand, is con- 

 fined to the larger cell, and obviously represents that part of the 

 substance of the fused polar lobe that appears at the surface, a 

 part having again moved into the interior of the egg.^ Upon 

 the 2-cell stage thus formed is moulded the entire subsequent 

 development, which in Its general outline Is of essentially the same 

 type as In such forms as Unio or Nereis. 



The experiments recorded In this paper relate mainly to the 

 significance of the material of the lower polar area, and of the 

 polar lobe, and form a continuation of those begun by Crampton 



1 Cf. Wheeler's Fig. 



