The time-graded regeneration field in planarians 



is very difficult to imagine that head-inducing capacities should exist at several cell 

 levels of the body when the wound faces forwards, but tail-inducing capacities at 

 the same levels when the wound faces backwards. 



It is not known what force imposes the time-graded field on the blastema, and 

 drives the cells at the high-point to differentiate more quickly. However it is possible 

 that crowding influences their metabolism, perhaps for instance through a deficiency 

 of oxygen. 



The blastema derives its polarity from the adult tissues. In the words of A. E. 

 Needham (1952) concerning the determination of the blastema, 'There remain a 

 number of morphogenetic difficulties, however, which cannot be explained by 

 Child's simple theory, and will ultimately demand a more sophisticated scheme with 

 more emphasis on qualitative gradients, and on multilateral competition, rather 

 than on unilateral dominance — a democracy rather than a monarchy. The demo- 

 cracy is not one of anarchic equality, but one in which every section normally ensures 

 its appropriate self-expression.' 



inhibihon 



Figure 21. Schematic representation of cell clusters in a 



transverse section of a blastema information. Explanation in 



text. 



It is now well established that the effects of genes become apparent — whether 

 morphologically, physiologically or biochemically — successively in time. This has 

 been shown by Hadorn (1948) and Poulson (1945) on lethal genes in Drosophila. 



In the following working hypothesis, the differentiation of the blastema is inter- 

 preted in terms of successive gene actions and of inhibitions exercised by the median 

 parts over the more peripheral parts of the blastema. Centrally placed neoblasts 

 (1 in Figure 21) differentiate more quickly, that is to say, their cytoplasm more 

 quickly reaches such a state as to respond to brain-determining gene actions. In 

 differentiating they inhibit the neighbouring cluster of cells (group 2) from respond- 

 ing to brain-forming gene actions. The next gene action in time is eye-determining, 

 and to this these adjacent cells now respond. Neoblasts later in the order of differenti- 

 ation are inhibited from forming brain and eye, and so form muscle (group 3), 

 intestine (group 4), or do not differentiate (group 5). These last remain totipotent. 

 This hypothesis is represented schematically in Figure 22. 



Geneticists are now laying more and more stress upon the conditions in the milieu 

 necessary for the unfolding of gene action. Beadle and others have stressed the import- 

 ance of the cytoplasmic state of single cells, and the work of Bonner (1952) and 

 Sussmam (1952) on the slime mould Dictyostelium suggests the same principle of 

 cytoplasmic reactivity. In this connexion we must clearly consider the conditions in 

 the time-graded morphogenetic field. 



135 



