H. V. BR0NDSTED 



I venture to suggest that serological incompatibility is at the root of negative 

 cytotaxis. I further venture to suggest that when a cell or a group of cells during 

 early morphogenesis has started determination in a fixed direction, it builds up a 

 serological equipment characteristic of the cell or group of cells, and thereby exer- 

 cises negative cytotaxis on neighbouring cells which have not yet reached a level 

 enabling them to start differentiation; these neighbouring cells are inhibited from 

 differentiating along the same lines. It should be possible to test this working hypo- 

 thesis by the explantation technique. 



According to this hypothesis inhibition is a question of serological nature; it is 

 transmitted from cell surface to cell surface. The response of a cell to the stimulus 



Figure 20. Spongilla lacustris. Photomicro- 

 graph of two cells touching one another; one is 

 giving off a pseudopodium with explosive force 

 (X 1,000). (Brondsted, 1936.) 



from its neighbour depends upon its reactive state. This in turn is determined by 

 the situation of the cell in the time-graded morphogenetic field. And this considera- 

 tion brings us to the concluding part of my paper. 



THE RELATION BETWEEN GENE-ACTION AND THE TIME-GRADED 

 MORPHOGENETIC FIELD 



It has always puzzled embryologists that cells presumed to contain the same genome 

 should differentiate differently. Even for this there is no conclusive proof, so long as 

 it is not possible to cause differentiated cells to dedifferentiate into a totipotent condi- 

 tion from which they might then regenerate all parts of a new organism, including 

 the germ cells. The same problem of differentiation exists both for embryos and for 

 regenerating blastemata. 



However it is difficult to transfer our notions of evocation and induction from 

 embryogenesis to morphogenesis in the blastema. In an embryo provision is made for 

 the unequal distribution of substances to the blastomeres, so that they are different 

 from the start. On the other hand in the blastema all the cells are at first alike, and 

 any induction or evocation must be initiated after the wound has been inflicted. It 



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