Invertebrates 



667 



tube containing a bubble of oxygen placed 

 over the end of the stem inhibits regenera- 

 tion (Rose and Rose, '41). This inhibition is 

 not cavised by a lack of oxygen, since oxygen 

 is supplied and therefore inhibition must 

 be caused by the accumulation of excretory 

 substances within the glass tube. In the 

 case of the ascidian stolon, either carbon 

 dioxide or urea or uric acid will inhibit 

 zooid formation but allow stolons to grow. 

 In summary, we may conclude that in 

 some organisms, at any rate, a svirface layer 

 prevents free diffusion of substances which 

 inhibit regeneration. When the surface layer 

 is removed by cutting, then carbon dioxide, 

 urea and uric acid escape more readily and 

 the exposed region is able to undergo regen- 

 eration. 



POTENCIES OF CELLS 



During early embryonic development an 

 important question is, In how many ways 

 can the various cells differentiate? Similarly 

 we may ask whether the cells participating 

 in regeneration have wide potencies or lim- 

 ited potencies. This problem is difficult to 

 solve and perhaps we can do little more here 

 than to state the problem. 



In the first place we may visualize a num- 

 ber of possibilities as regards the potencies 

 of the regenerating cells. 



1. Imagine that all of the cells at the cut 

 surface proliferate and form a mass of un- 

 differentiated cells with wide potencies. 

 Then some inductive influence from adjacent 

 structures must induce differentiation. 



2. The cells at the cut surface dediffer- 

 entiate morphologically but retain their 

 specificity as regards cell types. Then a 

 muscle cell which dedifferentiates must re- 

 differentiate into a muscle cell. Cells retain 

 their limited potencies. 



3. Cells at the cut surface do not prolifer- 

 ate but rather transform directly into the 

 regenerating structures. This process may or 

 may not involve changes in cell type. Some 

 cells must have a number of potencies to 

 supply missing structures. 



4. The cells at the cut surface take little 

 part in regeneration and the main source 

 is from undifferentiated cells which migrate 

 to the site of the wound. Missing structures 

 would thus differentiate from reserve cells 

 (embryonic cells, neoblasts, formative cells, 

 eleocytes) which are present throughout 

 adult tissues and have wide potencies. 



The whole problem of the origin of the 

 cells which differentiate during regenera- 



tion and the very important implications 

 in regard to a possible reversal of biochem- 

 ical differentiation will be a challenge to 

 investigators for some time. A histological 

 dedifferentiation is clearly shown by Hux- 

 ley's histological studies on ascidians. How 

 are we going to trace cells during regener- 

 ation? Can we trace a nerve cell from the 

 adult ascidian zooid into the stolon during 

 regression and then trace this same cell back 

 into the new zooid which forms from this 

 stolon? Does this nerve cell redifferentiate 

 into a nerve ganglion cell or may it form 

 an intestinal cell? Are cell types immutable? 

 For those who maintain the affirmative to 

 this latter question I would point out that 

 the atom was once considered immutable. 

 As to those who take the negative, I must 

 remind them that conclusive evidence of 

 a major change in cell type is still wanting. 



Certain facts are clear and must be con- 

 sidered before reaching a judgment on this 

 question. In forms such as Tubularia the 

 hydranth develops directly from the coeno- 

 sarc near the cut surface. There is no growth 

 and little cell division. A section of the stem 

 about 2 mm. in length transforms directly 

 and quickly (24 hours) into a hydranth. 

 There can be no question here of reserve 

 cells migrating in from the rest of the stem 

 and differentiating into a hydranth. Indeed, 

 a section of the stem 2 mm. long will trans- 

 form completely into a bipolar form with 

 a hydranth at each end. Nor is there any 

 proliferation of reserve cells accompanied 

 by a degeneration of cells of the stem. There- 

 fore a cell which is part of the stem is 

 converted directly into a cell forming part 

 of a tentacle. 



During the regression of an ascidian zooid 

 into the stolon, the latter becomes packed 

 with cells which dissociate from the tissues 

 of the zooid (Huxley, '26). These cells flow 

 through the circulatory system. They are 

 alive as witnessed by observation of the 

 living stolon and by histological examina- 

 tion. Therefore nerve, heart and intestinal 

 cells pass from the regressing zooid into the 

 stolon. This stolon may then form zooids 

 when cut into short sections. What happens 

 to the nerve, heart and intestinal cells when 

 the new zooid forms? Do they undergo 

 cytolysis or do they take part in the forma- 

 tion of the zooid? There is no evidence of 

 extensive cytolysis and thus the cells prob- 

 ably enter the new zooid. If they do, will 

 a former nerve cell enter the intestinal tract 

 and become a secretory cell or must it rediffer- 

 entiate into a nerve ^ell? This question cgn- 



