46 Edmund B. Wilson. 



ments developed. I have only one pair of camera sketches to 

 show the polar lobes in such a case (Fig. 75,76). In both these 

 the lobe is relatively too small, as if produced from insufficient 

 material; but this not always the case (as shown beyond), and 

 it should be remembered that the polar lobe is sometimes too 

 small even in a lower fragment containing the whole of the lower 

 polar area (Fig. 62,). Figs. 77a, 78a show a pair, one of which 

 has a lobe of normal proportions ; the »ther is a very nearly nor- 

 mally formed 2-cell stage, though the larger cell is perhaps a trifle 

 too small. Both these produced nearly normally proportioned 4- 

 cell stages (Figs. 77b, 78b). Several other cases, in which only 

 one fragment developed, showed a normal trefoil. These data are 

 somewhat meagre, yet they justify the conclusion, I believe, that 

 after vertical section bisecting the lower polar area both frag- 

 ments may segment like whole eggs of half size. 



The above conclusion renders it probable that by such vertical 

 section two perfect dwarf trochophores may be produced from 

 a single egg, which is apparently impossible when one fragment 

 alone contains the lower polar area. In point of fact, I have 

 never obtained even a single wholly normal larva after such sec- 

 tion; but in view of the comparatively small number of successful 

 operations and the very small number of such larvae obtained 

 by section in other planes this is not surprising. A number of 

 larvae from more or less nearly vertical sections is shown in the 

 following figures. Fig. 88 is a nearly normally formed larva 

 with two apical organs, from an oblique section passing outside 

 the lower white area. Fig. 89 is a nearly normal larva from a 

 section that removed a part of the lower area. Fig. 93 is from 

 an exactly vertical section bisecting both areas. In section this 

 larva Is closely similar to a normal one, and seems to show that 

 the trochoblasts are as large as in a whole embryo. Fig. 90 Is 

 from the smaller fragment after a slightly oblique section bi- 

 secting the lower area ; a very distinct apical organ is present and 

 also an abnormally formed post-trochal region. Figs. 91, 92 are 

 twins from a slightly unequal vertical section (developed from the 

 respective twin fragments 81, 82), the post-trochal region is 

 lacking In both, while one lacks an apical organ. 



