DEVELOPMENT IN PLANARIANS. 28 1 



active. E. Scholtz (op. /.) has given an excellent account of 

 the regeneration of the reproductive organs in D. lacteunt. 

 Morgan has shown (op. citJ) that in P. lugubris pieces cut anterior 

 to the old reproductive organs may regenerate them. 



An exceptionally good discussion of histogenesis in forms 

 allied to the planarians may be found in the article by Keller 



(op. /.) 



FACTORS DETERMINING DEVELOPMENT. 



Having thus considered the origin of the tissues entering into 

 the new parts, we may now consider the factors concerned in the 

 development of the various regions in planarians. Of these 

 regions the most definite and fundamental are the head and 

 pharynx. In the embryo the head is developed at one extremity 

 of the yolk-cavity, the pharynx at the other. Between these two 

 regions the yolk-cavity is converted by the ingrowth of septa 

 into an axial gut with lateral branches. The axial gut grows 

 forward into the developing head and posteriorly it sends two 

 branches into the caudal region which develops behind the 

 pharynx. 



In regeneration we likewise find that pharynx and head are 

 developed with certain relations to the intestinal system of the 

 isolated piece. Brought near to one another by muscular con- 

 traction isolated intestinal branches become anastomosed into a 

 unit system. By making certain cuts it is possible to have two 

 or more such unit systems developed in a single piece, as shown 

 in Fig. 12, B. Head and pharynx are developed in connection 

 with such a unit system only when there is contained within the 

 piece a portion of the central nervous system. This is shown in 

 Fig. 10, where the unit intestinal system of the anterior lateral 

 slip has neither head nor pharynx developed in connection with 

 it. Furthermore, the piece containing the unit system must be 

 above a minimum size. Thus in very short head pieces and 

 short, narrow lateral slips no pharynx may be developed al- 

 though the piece contain a portion of the original intestinal 

 system and a portion of the original nervous system. 



What determines the location of the anlage of the pharynx ? 

 In a previous paper (op. cit.) I showed that the pharynx arises in 

 a region posterior to that portion of the intestinal system into 



