loS 



KROEBER. 



[VOL. II. 



as sections of the small pieces removed showed, that ectoderm 

 and endoderm had met. In the majority of instances, however, 

 I was fortunate enough to remove the invaginating ectoderm 

 just in time. In cases where this was done with a hot needle 

 there is, of course, nothing to prove that the fusion had not 

 taken place. There is ground for such a belief, however, in 

 the fact that, of a number of worms whose small anterior ends 

 were cut off at the same time after the first operation as when 

 the burning was done, and which were kept under exactly the 

 same conditions, there was not a single one in which the fusion 



had taken place. 



The same difficulty 

 presents itself again in 

 determining the time at 

 which the worm is to 

 be killed. I succeeded, 

 however, in getting a 

 number of cases where, 

 though the pharynx and 

 the ectoderm were just 

 on the point of joining, 

 they had not quite done 

 so when the worm was 

 killed. The accompany- 

 ing figures show two 

 worms in this condition. 

 Fig. i shows a verti- 

 cal longitudinal section 

 of a worm from which seven segments were removed on 

 January 15. On February 2, that is to say eighteen days 

 later, the tip of the newly regenerated part was cut off. This 

 piece was preserved and sectioned and was found to include the 

 whole of the ectodermal invagination besides the anterior end 

 of the pharyngeal diverticulum which had not yet broken 

 through to the exterior. Fourteen clays after this operation, 

 on February 16, the worm was killed. 



Fig. 2 represents a vertical longitudinal section of a worm 

 from which the first seven segments were cut off on January 



FIG. i. 



