138 MORGAN. 



that the two are carried further and further apart. These results 

 show that the principal growing region in the new worms lies in 

 front of the pharynx, and it is conceivable that in the absence of 

 the pharynx this region of growth is not formed. 



The influence of the width of the region of attachment a fac- 

 tor to which I ascribed some influence in my last paper should 

 also be taken into account here. If a pharynx is present in the 

 part the area of union seems to play a less important role than 

 when there is no pharynx present. It is probable that it is not 

 simply the area of union as such that plays the important part, 

 but it is the connection between the internal organs possibly the 

 digestive tract that is chiefly involved in the result. 

 THE FORMATION OF HETEROMORPHIC HEADS IN PLANARIANS. 



The formation of a heteromorphic head in Planaria litgubris, 

 when the old head is cut off just behind the eyes, has been 

 described in several of my preceding papers. In the past sum- 

 mer I succeeded in obtaining one such case in Planaria maculata, 

 but only after a large number of trials, and furthermore in this 

 successful case the cut was not immediately behind the eyes, as 

 seen in Fig. 22. At the same time I cut a large number o' 

 worms into short cross-pieces, keeping all pieces of the same 

 length together. To my surprise I found that the only pieces 

 that produced a head at the posterior end, as well as at the 

 anterior end (Figs. 23 and 24) were those taken just behind the 

 old pharynx in the region of the reproductive pore. Two possi- 

 bilities suggest themselves, viz., the presence of the reproductive 

 organs in the piece, or the cut being made through the region at 

 which this planarian pinches off pieces of itself to form new 

 worms. If the posterior edge of the cross-piece lay in the 

 region of constriction, where the new head of the new worm 

 would develop, it is conceivable that this might cause the de- 

 velopment of the heteromorphic head in the cross-piece. A 

 similar series of cross-cuts were made on Planaria lugnbris, but 

 double-headed pieces were never formed on the cross-pieces from 

 the region of the reproductive pore. In this species, however, 

 new worms are not formed normally by pieces constricting off 

 from the posterior end. 



Neither of these possibilities seems to me to give a satisfactory 



