414 F. H. Krecker, 



intestine but begin to migrate only after it has reaclied a considerable 

 length, in fact it may be ahnest a segment long before the oldest portion 

 of tlie new intestine begins to be siirrounded by the chlorogogue. The 

 first Step in the process is an elongation of the chlorogogue in the neigh- 

 borhood of the point from which the regeneration of the intestine 

 originally started. This elongation may serve to cover the immediat- 

 ely adjoining new parts but as growth proceeds certain of the chloro- 

 gogue cells become detached and move down along the sides of the 

 intestine. Free migration in the coelom does not usually occur al- 

 though this may also take place. Most of the cells migrate by creeping 

 along the sides of the intestine. Normally the long axis of the chloro- 

 gogue cells is j^erpendicular to that of the intestine but those cells 

 that migrate along its sides gradually change their shape. The former 

 longitudinal axis is shortened while elongation takes place in the oppo- 

 site direction until finally the cell is extremely attenuated and flat- 

 tened against the intestine. In the meantime it moves toward the 

 posterior end (Fig. 8). 



The Proctodaeum. 



There has been considerable discussion and no little disagreement 

 among the numerous writers on the subject of posterior regeneration 

 as to just how the anal opening is formed. Some authors maintain 

 that there is a proctodaeal invagination while others deny it. Ran- 

 DOLPH (92), V. Kennell (82) and v. Zeppelin (83) describe the for- 

 mation of a proctodaeum similar to that found in embryonic deve- 

 lopment. Rievel (96) and v. Wagner (06) claim it is formed from 

 ectoderm alone, on the otlier hand Hepke (97) and Haase (98) main- 

 tain that it is composed of entoderm v/hile v. Bock (98) and Michel (98) 

 found a simple fusion between the body wall and the intestine without 

 the closing of the aperture. Abel (02) seems to have harmonized 

 the various views by finding that the method differed even in the 

 same species; both intestine and body wall may closc then meet and 

 form an opening or eise, and particularly at levels near the anterior 

 end, the sinking in of the ectoderm comes before the opening is formed 

 and lastly both intestine and body wall may remain open and merely 

 fuse, this being the case at the more posterior levels. 



In the present instance where a portion of the intestine has been 

 removed a proctodaeal, or perhaps better a pseudoproctodaeal in- 

 vaginatioji is of frequent occurrence, but one must be careful to dis- 

 tinguish between what can be considered even a pseudoproctodaeal 



