DEVELOPMENT IN PLANARIANS. 2/5 % 



gions are likewise developed in a manner resembling the em- 

 bryonic. 



If the cut surface on which the new head is developed be at 

 right angles to the long axis of the parent worm the new head will 

 be developed symmetrically about the anterior extremity of the 

 axial gut, just as it is developed symmetrically about the anterior 

 extremity of the yolk-cavity in the embryo. If, however, the cut 

 be made oblique, the head may arise some distance anterior and 

 lateral to the region where the old axial gut projects on the cut 

 surface. Morgan first described this phenomenon. 



A B 



P'IG. 8. To illustrate the development of a new individual from a cross piece 48 

 hours after removal. Magnification, 57 diameters. 



A. Individual as seen from the dorsal surface. The intestinal system was 

 sketched from the living individual, the nervous system was reconstructed from 

 sections. 



B. Sagittal section in the median line from the same individual, ph-p-, Fharyn- 

 geal pocket. 



Fig. 9 shows a good example from Dcndroccehnn lacteuui. 

 The median ends of the lateral intestinal branches lying in the 

 piece anterior to the extremity of the old axial gut and separated 

 from the latter by the cut have become so anastomosed as to 

 form an extension of the old axial gut. About the anterior ex- 

 tremity of this the new head is being symmetrically differentiated. 

 In the further growth of the piece new tissue is rapidly formed 

 to the right of the axial gut and about its extremity until the 

 worm is restored to normal proportions. 



Tail Pieces. In tail pieces regeneration of the new head takes 

 place symmetrically about a branch extending forward from the 

 area effusion of the two posterior rami of the original piece. In 

 all of the instances which I have examined this fusion has taken 

 place before differentiation of a head has really begun. The 

 process of head formation resembles essentially that described as 

 taking place on the anterior surface of the cross piece taken from 

 in front of the pharyngeal region. The formation of the pharynx 



