DEVELOPMENT IN PLANARIANS. 283 



In PJiagocata gracilis F. R. Lillie L has shown that in the 

 pharyngeal anlage a considerable number of pharyngeal pouches 

 are developed and that these subsequently fuse. In lateral slips 

 the pharyngeal pouches on the cut side are formed only subse- 

 quently to the development of an intestinal branch on that side. 

 In this animal the pharyngeal area seems to be determined by 

 factors similar in nature to those at play in Planaria metadata, 

 but pharyngeal formation in this area is limited not to a single 

 stimulus exerted posteriorly from the region of the axial gut, 

 but, on the contrary, may be excited by stimuli arising from 

 anterior median branches of the chief posterior rami of the devel- 

 oping intestinal system. For a criticism of the view which I have 

 expressed as to the factors determining the seat of pharyngeal 

 formation the reader is referred to Lillie's article. 



The pharynx may be formed either near a cut surface or quite 

 within the body of the piece. A new head is formed only in the 

 tissue produced on a cut surface. This surface may be a flat 

 plane. In such a case the median axis of the head will at first 

 be perpendicular to the cut surface. Or the head may be pro- 

 duced at the junction of two planes. Thus, if the anterior end 

 of the worm be removed by a V-shaped cut, the angle of the V 

 pointing posteriorly, and the two sides be allowed to unite, as a 

 rule a new head will be produced, the median axis of which is 

 in line with the chief axis of the worm. If they be not allowed 

 to unite a head will be produced on each side with its median 

 axis perpendicular to the plane of the cut. If the V point an- 

 teriorly sometimes a head will be produced on each cut surface ; 

 at other times a single head will arise, the median axis of which 

 corresponds with the longitudinal axis of the worm. When a 

 new head arises thus at the juncture of two plane surfaces the 

 median axis of the head seems to bisect the angle formed by the 

 junction of the two cut surfaces. Of course, the cut surfaces may 

 not be planes, but may be very irregular in shape. In all such 

 instances, however, similar axial relations prevail. 



The direct stimulus to head and brain formation seems to be 

 due to the presence near the cut surface 01 the chief coordi- 



1 "Notes on Regeneration and Regulation in Planarians, American Journal oj 

 Physiology, Vol. VI., p. 134, 1901." 



