NOTES ON REGULATION IN STYLARIA LACUSTRIS. 183 



Planaria, with its diffuse excretory system, could not be expected 

 to occur in a form like Stylaria, with a head region so specialized 

 as to be unable to exist independently even for a short time. 



A case of duplication of parts was seen in the production of 

 a double-headed individual, one of whose heads was larger and 

 in line with the axis of the trunk. The other was lateral, lying 

 close to it, and smaller. The smaller one was in process of 

 absorption at the last stage at which it was observed (Fig. 8). 



A doubling of the prostomium was noticed in one case as 

 mentioned above. The two organs lay parallel and were fused 

 together except at the tip of the shorter (Fig. 9). In later stages 

 the lateral bud was absorbed. 



9. Regulation of growths oblique to the main axis. 



It is a common observation that proliferation tends to take 

 place along an axis normal to the cut surface. After an oblique 

 cut the new growth continues oblique until it has reached its 

 full length or nearly so. The straightening of such oblique 

 growths is evidently not a function of growth alone, but of growth 

 influenced by certain tensions, which may be exerted by the part 

 behind the new growth. Rievel ('96) has pointed out the influ- 

 ence exerted by the peristaltic motions of the intestine upon the 

 regeneration of the head region. The knob of new tissue is at 

 first solid and is later penetrated by the slower ingrowth of the 

 lumen of the intestine into the region. The waves of peristaltic 

 motion constantly passing along the intestine favor its ingrowth 

 into the new region. Finally the mouth is formed by the 

 bulging out of the body-wall, caused by peristalsis. It is broken 

 through from within, there being no stomodaeal invagination of 

 the ectoderm in regeneration. 



It may be suggested that peristaltic movements of the intestine 

 have an influence in straightening oblique growths. This is 

 probable since the straightening occurs not during the prolifer- 

 ating period, but coincidently with the ingrowth of the pharynx. 

 Straightening occurs, moreover, while the tissue still appears 

 embryonic before the complete differentiation of muscular and 

 nervous systems (Figs. 10 and 13). 



