2464 Plathelminthes : III. Turbellaria. 



Am ,,Leitthier" wachst das Integument des Hinterendes nach vollzogener 

 Wuudheilung zur Bildung des neu zu beschaffenden Caudalauhangs nach 

 hinten aus. Aelmlich lauten die Angaben Ott's (648, pag. 296) bezug- 

 lich S. leucops, indem auch nach diesem Autor sich zimachst die epider- 

 moidale Riugfurche mehr und mehr vertieft until it touche the intestine". 

 Ott fahrt dannfort: ,,The wall of the intestine also becomes constricted 

 at the same time. The intestine affords the last connection between the 

 mother and the bud. When the constriction of its wall closes this 

 connection, the bud fastens itself by its posterior end to some object, 

 and the mother, by a few quick contractions, separates itself from it. 

 The integument soon grows over the anterior end of the bud, and over 

 the posterior end of the mother. The integument at the posterior end 

 of the mother continually grows backward until a new tail is formed." 

 Child (969, pag. 198) berichtet sehr ausfuhrlich iiber die Kingfurchen- 

 bildung und Ablosung bei S. grande und leucops und gelaugt dabei zu 

 einer von der oben reproducirteu Auffassung Keller's betrachtlich ab- 

 weichenden Ansicht iiber die Natur der betreffeuden Vorgange. Hier 

 kann nur das Weseutlichste mitgetheilt werden; bezuglich der von Child 

 zuni Beweise seiner Aufstellungen herangezogenen Beobachtungen uud 

 Erfahrungen muss auf das Original (969) verwiesen werden. ,,The con- 

 striction" - - sagt Child ,,appears a short distance in front of the 

 forming ciliated pits, being at first very slight and visible only in the 

 contracted condition but becoming more and more sharply marked as 

 development proceeds. Keller believes the formation of the constriction 

 to be a purely muscular process due to stimulation of the circular muscles 

 of the body-wall and alimentary canal, causing constantly increasing tonic 

 contraction and finally the separation at the fission-plane. Judging from 

 the form of the alimentary canal in the region of the fission-plane at 

 the various stages, I am inclined to the view that the constriction of the 

 alimentary canal is largely or wholly the mechanical result of the pressure 

 exercised upon it by the growth of the brain and pharynx and the con- 

 traction in the body-wall". Und weiter unten heisst es: ,,According to 

 Keller the separation or dissection at the fission-plane is the result of a 

 mechanical breaking up of the intimate connections of the tissues in the 

 region of the fission plane, this in turn being the result of the extreme 

 muscular contraction. These tissues are in position to form a new head, 

 and in consequence of this position the stimuli affecting them produce 

 increasing physiological isolation from the tissues anterior to them and 

 finally separation. In short this region has been specified as a head- 

 forming region, and as such it possesses certain physical or physio- 

 logical chemical qualities or both which result in its isolation from 

 tissues anterior to it. Just what these qualities are must be left unde- 

 termined for the present, but cases of constriction and separation of 

 parts occurring as the result of changes of this kind without anything 

 approaching muscular contraction are common enough to convince us that 



