52 Edmund B. JVilson. 



a fertilized egg is thus sectioned only the nucleated {i. e., the 

 upper) fragment develops — a result that agrees with my ob- 

 servations on the nemertine egg and that of Renilla ('03), and 

 with the earlier ones of Delage ('01) on those of echinoderms. 

 This fragment has essentially the same mode of development 

 as a corresponding fragment of an unfertilized egg, segmenting 

 equally into two and four without the formation of a polar lobe, 

 forming successive symmetrical quartets of micromeres by alter- 

 nating spiral cleavages (Fig, 95), and producing a larva that is 

 either an irregular monster or a pyriform larva closely similar 

 to those arising from the lobeless egg or the AB half. This is 

 what would be expected in view of the preceding results; but 

 the behavior of the non-nucleated lower half is most remarkable 

 in that it forms three times in succession a polar lobe from the 

 white area at the same time that the nucleated half is dividing^ 

 becoming spherical after each period of activity without dividing. 

 When this was first observed, I believed that I must in some way 

 have confused the fragments with those of unfertilized eggs; but 

 repetitions of the experiment under conditions that precluded all 

 error, gave the same result. A typical case is shown in Fig. 94, 

 from consecutive camera drawings of the same fragment. The 

 first lobe is shown (Fig. 94b ) about 15 minutes after the opera- 

 tion, while the nucleated half (Fig. 95) has just divided into 

 equal halves. Twenty-three minutes later the fragment was again 

 perfectly spherical (94c), while the upper fragment was in a 

 resting 2-cell stage. The second lobe (94d) was formed 44 

 minutes after the first, while the upper fragment was dividing into 

 4 equal cells, after which the lower fragment again became spher- 

 ical (94e, 16 minutes later than 94d). The third lobe (94f) 

 was formed 32 minutes after the second, and was considerably 

 smaller than either the first or the second, as in a whole egg; 

 the upper fragment meanwhile divided into eight cells (Fig. 95b) . 

 A third period of rest followed (Fig. 94g) . Following the fourth 

 cleavage of the upper fragment the lower one passed through a 

 change no less remarkable than the preceding (it is at this 

 period in the normal development that a large part of the 

 lower polar area passes into the first somatoblast) . This 



