Soino Plipnoiiu'na of Regeneration in Liuinodriln?^ and related Fonns. 415 



invaginatiüu ajid a niere folding of tlie body wall. The latter also 

 often occurs, in fact there is some slight folding in nearly every indi- 

 vidual. Tlie foldings whicli can be termed pseudoproctodaeal occur 

 at the point wliere the cut edges liave met and extend inward in an 

 anterior direction for various distances, sometimes quite far. Their 

 edges may be straight or rough and puckered (Fig. 7). They are 

 the result of the same infhiences that cause the entire body wall to 

 fold over toward the median line, namely the strong contraction of 

 the body musculature, only in this case the process has gone farther. 

 In normal regeneration the presence and the posterior growth of the 

 iiitestine prevent such deep invaginations biit when the intestine is 

 absent there is no siipport for the weakened portion of the body wall 

 about the wound. Especially is this true in those cases where the 

 nmscle fibres immediately surrounding the wound have left to form 

 the straiid. The epidermis is then left without adequate support and is 

 unable to withstand the contiimed strain exerted lipon it by the contrac- 

 tion of the adjacent muscles and consequently the wall collapses at 

 this point and forms a deep de^Dression. 



An actual proctodaeum is formed only when the intestine reaches 

 the end of the body and fuses with the epidermis preparatory to the 

 formation of an anal opening. The epidermis in this region becomes 

 considerably thickened in the absence of the intestine and some cells 

 even migrate into the coelom but since the migrated cells lie along 

 the ventral side they present no obstacles to the growth of the intes- 

 tine and since the thickening is also mostly confined to the ventral 

 half of the body the epidermis at the point at which the intestine 

 touches it is usually of little more than the normal thickness. The 

 nianner in which the opening is formed corresponds with what has 

 been described heretofore by v. Kennel (82) and v. Zeppelin (83) 

 in the fission of Ckaetogaster and by Randolph (92) in the normal 

 regeneration of Lumbriculus as well as by Abel (02) in Tvbifex. As 

 mentioned before Abel finds that the anal opening is formed in three 

 chief ways. The method in this case is that. described by him under 

 the subhead »Die Regeneration des Enddarms mit Verschluß des 

 Darmes und Neubildung von Segmenten (Proctodäum)«. When 

 the intestine meets the epidermis the two fuse so that there is no sharp 

 line of demarcation between tliem. The entoderm is considerably 

 lighter in color than the ectoderm but even this distinction is not so 

 marked at the point of immediate contact since here the ectoderm 

 does not take the stain (hematoxylin) so readily as do the adjacent 



Zeitschrift t". wissensch. Zoologie. XCV. Bd. 27 



