RECONSTITUTIONAL PATTERNS IN EXPERIMENT 407 



body-level and physiological condition of animals; and that section of the 

 nerve is almost as effective in inhibiting the head as section across the 

 whole body. Apparently the inhibiting factor is, at least in large part, 

 nervous in character, a stimulation resulting from section of the nerve 

 cords at the posterior end of the piece (Watanabe, 19356) and probably 

 from effects upon them of the activated tissue there. Since active tissue 

 and nervous system are apparently the chief factors in dominance in the 

 mature planarian and the inhibiting factor also appears to be nervous, 

 the conclusion seems justified that it acts by decreasing the independence 

 of the cells concerned in head formation. 



At first glance it may seem improbable that a nervous stimulus from 

 more posterior levels can inhibit head regeneration, but a brief analysis 

 will show the grounds for this conclusion. It is evident that the cells at 

 any body-level are determined and continue to function as that particular 

 body-level only as long as they represent a certain relative gradient-level 

 and are in physiological relation with other parts, chiefly the dominant 

 region. This determination is reversible; and when they are isolated from 

 dominance, every body-level can become a dominant region and give rise 

 to a head and so determine reorganization of other parts. In other words, 

 every body-level will develop cephalic ganglia and a head when freed from 

 dominance and relations with other parts. Head regeneration is a self- 

 differentiation, and nervous tissue is apparently the primary differentia- 

 tion in head development. In fact, planarian parenchyma cells in tissue 

 culture give rise to long fibers and become indistinguishable from cultures 

 of ganglionic cells (Murray, 193 1). Nervous stimuli from other parts of 

 the body than the dominant head region probably play some part in 

 maintaining the characteristics of a particular body-level. When a given 

 level is isolated by section from more anterior levels, the cells adjoining 

 the level of section tend to lose their characteristics as cells of a particular 

 level, undergo a change in condition, and become a new dominant region. 

 But if another section is made posterior to the first, but within a certain 

 distance from it, the stimuli resulting from section of the nerve cords and 

 the activation of cells adjoining the posterior cut end tend to maintain 

 the characteristics of the body-level represented by the cells at the an- 

 terior end of the piece. They represent factors in the physiological rela- 

 tions of these cells with other parts and, while not sufficient to prevent 

 head formation in the absence of posterior section, are evidently intensi- 

 fied by such section and become effective in partly or completely inhibiting 

 head regeneration because their effect is to keep the cells functional parts 



