106 KROEBER. [VOL. II. 



at the point where this was cut, and that no ectodermal invagi- 

 nation whatever occurs, the endodermal diverticulum joining 

 the body wall to form the mouth at the very anterior end of 

 the worm. 1 



Haase showed that in Tubifex, when four to six anterior 

 segments have been removed, the pharynx grows forward out 

 of the walls of the digestive tract and meets an ectodermal 

 invagination of somewhat varying size. This ectodermal pouch, 

 which forms the buccal cavity, is small in all cases, never 

 extending quite as far back even as the region of the cerebral 

 ganglion. 



Von Wagner's observations on Lumbriculus show that the 

 point of union of ectoderm and endoderm, originally at the 

 anterior end of the animal, subsequently comes to lie more 

 posteriorly, on account of the forward growth of the " Kop- 

 flappen " and accompanying turning in of the ectoderm. 



The differences in the accounts cited above show clearly 

 that it is almost impossible to determine with certainty, merely 

 by observation, just how much of the regenerated pharynx 

 ultimately arises from the ectoderm and how much from the 

 endoderm. It is very easy to see where ectoderm and endo- 

 derm meet, but the point of fusion is lost soon afterwards, and 

 since the regenerated head continues to increase in size, it is 

 presumably possible that the point of union may come to lie at 

 some distance from its original position. At the time when 

 the pharynx opens to the exterior its walls are not sufficiently 

 developed for one to be able to determine whether the muscles 

 will grow around the endodermal part of the tube ; but if in 

 some manner the fusion of the ectoderm with the endoderm 

 could be delayed long enough for the pharyngeal muscles to 

 form around the latter, then the origin of the pharynx might 

 be determined. Hescheler affirms that it is possible to make 

 out the exact limits of the ectoderm by using stains which 

 bring out the cuticle covering this layer. I used the stain 

 which Hescheler mentions as giving the best results, but found 

 that, while my preparations showed in general an agreement 



1 For criticism of Rievel's results, see papers by Morgan (Roux's Archiv, 

 Bd. v, 1897) and Hescheler (Jenaische Zeitsckrift, 1898). 



