668 



Regeneration 



not be answered definitely. Recall, however, 

 that isolated parts of the adult zooid will 

 regress and form a mass of morphologically 

 dedifferentiated cells. This mass can then 

 form a complete zooid of reduced size. In 

 this instance when a region lacking the 

 nerve ganglion is isolated, where does the 

 new nerve ganglion come from? I would 

 like to urge a study of ascidian cell types 

 and their behavior during regression of the 

 zooids, during dedifferentiation of the stolon 

 and during regeneration of the parts of the 

 zooid. This material is relatively easy to 

 handle and changes are relatively rapid, an 

 important consideration where so much ex- 

 ploratory research still remains. Consulta- 

 tion of the papers of Driesch ('06), Spek 

 ('27), Huxley ('26), Deviney ('34), Berrill 

 and Cohen ('36), and Goldin ('48) will be 

 valuable in this connection. 



Another form which appears admirable 

 for studies on potencies of cells is the sponge. 

 Here cells may be dissociated and they will 

 aggregate and reconstitute a sponge. The 

 nature of the cells which take part in the 

 formation of the new sponge is still a matter 

 for investigation (Galtsoff, '25; Wilson and 

 Penney, '30; Penney, '33; de Laubenfels, 

 '34). However, since the adult sponge may 

 be broken up into individual cells, there 

 may be a means of separating the various 

 cell types and examining each type for its 

 ability to differentiate. If the various cell 

 types differ by some property such as spe- 

 cific gravity, then centrifuging the dissoci- 

 ated cells in sugar solutions would separate 

 the cells into layers. 



CORRELATIVE DIFFERENTIATION 



As in embryonic development the differ- 

 entiation of tissues during regeneration de- 

 pends on factors located in adjacent tissues. 

 The organizer phenomenon has been inves- 

 tigated in flatworms (Santos, '31; Miller, 

 '38) and in coelenterates (Child, '29; Li 

 and Yao, '45). In Dugesia the head region 

 acts as an organizer inducing changes in the 

 surrounding tissues. A small fragment of 

 the head region transplanted to the body 

 region will induce an outgrowth that de- 

 velops into a head and in addition induces 

 the host tissue to form a pharynx. What are 

 the similarities between the organizer in 

 Dugesia and the amphibian organizer? 1. 

 A small piece of transplanted tissue induces 

 a large part of the host to differentiate. 2. 

 Dugesia organizer is not species specific and 

 heteroplastic induction occurs. 3. The organ- 



izer is localized. In planarians it is restricted 

 to the head region while in an amphibian 

 gastrula only cells from the dorsal and lat- 

 eral lips of the blastopore will induce. 



In flatworms and annelids the nervous 

 system may play a special role in regenera- 

 tion. The nerve tissue appears to act as an 

 organizer for head structures. However, re- 

 generation of heads will still occur in absence 

 of nerve cord at the cut surface. These ob- 

 servations are in keeping with the properties 

 of the amphibian organizer phenomenon 

 where, although nerve tissue is a good organ- 

 izer, the chorda-mesoderm is the primary 

 organizer. For a discussion of the role of the 

 nervous system in various invertebrates, see 

 Child ('42, pp. 338-341). 



Another expression of correlative differ- 

 entiation is found in the regeneration of the 

 hydranth in Tubularia. If a piece of stem 

 is isolated, the apical (distal) cut end differ- 

 entiates into a hydranth while the basal 

 (proximal) cut end forms a stolon. The 

 apical end exercises an inhibition over the 

 regeneration of the basal end, a fact which 

 can easily be demonstrated by preventing 

 the apical end from regenerating with the 

 result that the basal end then regenerates 

 a hydranth instead of a stolon. Thus differ- 

 entiation of the basal end into stolon or 

 hydranth is correlated with the presence or 

 absence of differentiation at the apical end. 



The mechanism by which the regener- 

 ating apical end influences basal regenera- 

 tion has been investigated from two points 

 of view, that of electrical differences in 

 potential and that of competition. 



1. Correlations have been shown to exist 

 between the electrical differences in potential 

 and the behavior of the two cut surfaces of 

 the hydroid stem (see Child, '42, and Lund 

 and others, '47, for literature). If electrodes 

 are placed on the two ends of the stem a 

 P.D. can be measured. If a constant electric 

 current is passed through sea water contain- 

 ing stems, regeneration is inhibited at one 

 pole but normal hydranths regenerate at 

 the opposite pole. Thus an electric current 

 may determine polarity. The question as to 

 whether the electrical potential difference 

 measured between the two ends gives rise 

 to a current within the stem of sufficient 

 intensity to account for the inhibition exer- 

 cised by the distal end over the proximal 

 end is still a matter for investigation. The 

 possibility that P.D.'s are effective in other 

 forms such as the earthworm has been more 

 recently studied by Moment ('46). 



The general hypothesis of the E.D.P. as 



