H. V. BR0NDSTED 



Various obvious simplifications have been made in this discussion; interaction of 

 genes has been left out of consideration; and those who study such phenomena as eye 

 colour in Drosophila will stress the complexity of gene action and the influence of 

 hormones. However, planarians are very primitive organisms, and are not so far re- 

 moved from cell colonies as to make impossible the idea that in them genes act to a 

 large extent directly upon the cytoplasm of the neoblasts. 



The time-graded morphogenetic field and the gene actions are delicately balanced, 

 and it is not surprising that irregularities should sometimes occur. For instance, 

 occasionally the cell cluster to the left of the brain (group 3 in Figure 2 1 ) forms eye 

 tissue instead of muscle. Several extra eyes may be produced under the influence of 

 LiCl (Brondsted, 1942). Waddington (1947, p. 47) has pointed out that 'during a 



eye 



0ENE 



totipotent stage 



Figure 22. Schematic representation of the fate of a 

 cell lying either in level 1. 2, 3, 4 or 5 of the time- 

 graded field as set forth in Figure 21. 



period of competence there may be more than two alternatives open to a tissue'. 

 The heteromorphoses reported by Waddington (1947) affecting the antenna of 

 Drosophila and the well-known cases in Crustacea and primitive insects affecting leg, 

 eye-stalk, and antennal regeneration, may be due to a similar lack of adjustment, 

 although in these higher organisms hormones probably play a major role in the pro- 

 cess of differentiation. 



The postulated time-graded field may also break down in the egg. At least I am 

 strongly inclined to interpret Witschi's (1922, 1952) well-known experiments with 

 teratomes arising from over-ripe eggs as a breakdown of the field. 



It would be of great interest to grow an isolated blastema, and see what it produces. 

 So far this has not been possible, because it has always cytolysed. 



In further investigations we are especially interested in biochemical aspects of 

 regeneration, including amino-acids in various parts of the blastema, and the effects 



136 



