PHYSIOLOGY OF RECONSTITUTION OF PLANARIA LATA. 125 



in advanced starvation may remain intact longer than the body 

 wall. 



In general, the course of disintegration in this species is very 

 similar to that in P. dorotocephala, but some minor differences 

 appear. In long individuals of the latter, possessing more than 

 one zooid in the posterior region, the different zooids usually show- 

 different susceptibilities (Child, 'nc, '136). The early disinte- 

 gration of the region anterior to the posterior zooid in P. lata may 

 indicate the presence here of another zooid at a very early stage ; 

 in other words, some degree of physiological isolation with increase 

 of metabolic activity may exist anterior to the level of fission and 

 yet be insufficient to permit fission at this level. 



In P. dorotocephala the margins of the body always disintegrate 

 in KNC before median regions, while in P. lata this may or may 

 not be the case. These differences are apparently associated with 

 the specialization of the margins as motor organs, and particularly 

 as organs of secretion of slime. There are many glands in this 

 region and the alkaline cyanide stimulates these to increased ac- 

 tivity. Often the separate glands disintegrate before other parts 

 of the margins, particularly in P. dorotocephala. Apparently the 

 margins of P. lata are less highly specialized in this way, for the 

 glands appear less distinctly as regions of disintegration and there 

 is less motor activity of the margins. In neutral or acid cyanide 

 and in other acid agents the glandular activity is apparently not 

 increased and motor activity is decreased, and in such solutions 

 the susceptibility of the margins, even in P. dorotocephala, is usu- 

 ally less than that of median regions. 



In P. dorotocephala the dorsal body wall disintegrates earlier 

 than the ventral in alkaline cyanide. The dorsal wall is thinner 

 than the ventral and shows many localized regions of disintegra- 

 tion, apparently glandular. In acid agents ventral regions are 

 usually more susceptible. In P. lata the specialization of the dorsal 

 surface is apparently less, so that even in alkaline agents the differ- 

 ence between dorsal and ventral is slight and the ventral surface 

 is usually the more susceptible. 



In the early developmental stages of turbellaria the median ven- 

 tral region is more susceptible than lateral and dorsal regions 

 (Child, unpublished), and in full-grown planarians the new tissue 



