42 Edmund B. Wilson. 



and (within rather wide limits) whatever its size, this fragment 

 may segment in every detail like an entire egg of diminished 

 size, forming the polar lobe in normal fashion, and may give rise 

 to a dwarf larva nearly or quite normal in form and possessing 

 an apical organ. The study of a large number of these fragments 

 shows that while there is considerable variation in the size of the 

 polar lobe it is as a rule of approximately and often exactly, 

 of the correct proportional volume; and this is true eveij after a 

 horizontal section that. passes quite outside the limits of the polar 

 area. By varying the plane of section it is thus possble to obtain 

 a graduated series of forms leading down from a full-sized em- 

 bryo to one not more than one-fourth this size, the fragments from 

 the other halves forming a similar series grading in the opposite di- 

 rection. That the form of cleavage Is within wide limits, inde- 

 pendent of the size of the piece, is thus strikingly demonstrated. 

 Such a graded series of trefoils and the corresponding equal 

 2-cell stages, is shown in Figs. 52-60, the last of these showing 

 the smallest one observed. Regulation of the size of the polar 

 lobe sometimes fails however, examples being shown in Fig. 63, 

 where, even after horizontal section, the lobe is too small (this 

 egg produced a larvae possessing an apical organ, but with the 

 post-trochal region greatly reduced). Fig. G^a, where it is slightly 

 too large, and Fig. 66^ where it is much too large; but these are 

 exceptional. It is hardly possible that this apparent regulation 

 is owing to the fact that the specific polar material extends so 

 far up into the interior of the egg that a section in almost any 

 plane includes the right amount of material to form a normally 

 proportional lobe. Such an explanation is rendered very improb- 

 able by the usual failure of the upper fragment to form a lobe 

 even after horizontal section far down in the vegetative hemi- 

 sphere or after oblique section; and still more improbable by the 

 fact that so many of the fragments form a normally proportioned 

 lobe, whatever be the plane of section. The conclusion therefore 

 appears unavoidable that the size of the polar lobe, and hence 

 of the structures dependent upon it, is subject to a regulative 

 process, from which it follows that the predetermination of the 

 region of the polar lobe is qualitative, not quantitative, or if 



