282 CHARLES RUSSELL BARDEEN. 



which, on general bodily contraction, the contents of the whole 

 intestinal system tend to be forced. I called this portion of the 

 intestinal system the area of "least intestinal pressure" but this 

 term is bad because it admits of several interpretations. The 

 area into which the intestinal contents tend, on general contrac- 

 tion, to be forced, and in which the new pharynx arises, is essen- 

 tially the area of least bodily compression. In P. macidata the 

 stimulus to pharyngeal formation is exerted in an antero-posterior 

 direction in this region. Polarity in the piece is determined by 

 the central nervous system. The pharyngeal anlage first appears 

 near the ventral wall of the body. 



In the embiyo it seems fair to assume that the contents of the 

 yolk-cavity are forced toward the embryo-pharynx on general 

 contraction. Closure of this would cause a stimulus to pharynx 

 formation to be exerted on the body wall comparable to that 

 exerted in regenerative development. 



In cross pieces taken from in front of the pharynx and in lat- 

 eral slips the relation of pharynx to intestines is fairly clear (Fig. 

 8 and Fig. 1 1). 



In tail pieces the problem is more complicated. When no re- 

 productive organs are present the pressure exerted within the 

 anterior extremities of the posterior intestinal rami by contrac- 

 tion of the piece tends to force the intestinal contents into the 

 anterior median branches of the posterior rami, and thus to give 

 rise to an intestinal area not at first occupied by a single intestinal 

 cavity, or axial gut, but by several distended branches. Posterior 

 to this area pharyngeal formation may be started even before the 

 median branches of the posterior rami have fused to form an 

 axial gut. When, however, a portion of the pharyngeal pocket 

 or the sexual organs are contained in the piece the anterior 

 median branches of the posterior rami are thereby prevented 

 from expanding so that the intestinal contents are forced further 

 forward into the area anterior to the pharyngeal pocket or to the 

 reproductive organs. 



The exact nature of the stimulus giving rise to pharyngeal 

 formation is still in the dark, although we can point out the area 

 where it seems to be exerted. It is fairly certain, however, that 

 bodily fluids are forced in considerable quantity into the area of 

 the pharyngeal anlage. 



