DEVELOPMENT IN PLANARIANS. 285 



it. Subsequently, normal form is usually restored by forward 

 growth of the head region and absorption of the lateral slip. By 

 making two such anterior lateral slips, one on each side, it is 

 possible to get a head to develop posterior to a ring of tissue 

 formed by the anastomosis of these slips. As the head grows 

 it projects dorsally in a forward direction beyond the anastomos- 

 ing lateral slips. 



The influence exerted by the intestinal apparatus upon the 

 regenerating part is probably in the main a nutritive one. Owing 

 to the action of the musculature upon the intestinal system that 

 region of the periphery of the body which receives the richest 

 nutritive supply is probably the area about the tip of the axial 

 gut where the head is differentiated. Owing to its proximity to 

 the axial gut the whole anterior regenerating region also receives 

 a rich nutritive supply and this may account for its rapid growth 

 until normal proportions are reached. 



The effect of proximity to the axial gut on regenerating tissue 

 is illustrated by the following experiment. 



If a narrow longitudinal slip of tissue be removed from one 

 side of the anterior half of the body and a broad slip from the 

 other, regeneration of tissues will be the more rapid on that side 

 from which the broad slip has been removed. This area of more 

 rapid regeneration lies nearer the axial gut. 



Morgan (pp. cit., p. 200) gives a picture of an oblique cross 

 piece in which the anlage of a new head has appeared before in- 

 testinal anastomosis is visible between the pharyngeal area and 

 the base of the head. Morgan traced the outlines of the intes- 

 tinal system of the piece from the pigment particles contained 

 within the intestinal cells. I have previously shown, however 

 (pp. cit., p. 22), that but little pigment is found in the newly 

 formed anastomotic intestinal branches, so that in the piece men- 

 tioned by Morgan an anastomotic axial gut may have existed. 

 By using small, slightly pigmented, starved worms in which the 

 reproductive organs are not developed the intestinal outlines may 

 be fairly accurately followed during life, if the pressure of a cover- 

 glass be used to flatten the specimens. Oblique illumination is 

 often of aid. In the hundreds of generating pieces of planarian 

 which I have examined, I have never seen an instance in which 



