472 J. RUNNSTROM 



laid, however, on the pathways that are responsible for the animal and 

 vegetal trends of differentiation. The scheme will indicate that the dif- 

 ferences between the different embryonic levels are primarily of a quanti- 

 tative rather than of a qualitative nature. One of our simplifications is 

 certainly to refer to animal or vegetal pathways instead of to families of 

 animal and vegetal pathways. It is to assume that each single pathway 

 corresponds to the formation of one specific ribonucleic acid. The intensity 

 of its formation is, however, regulated in the system. According to our 

 view the primary control is exerted by an animal and a vegetal cytoplasmic 

 centre. Each of these produce certain agents which spread in the direction 

 of the opposite pole, cf. [21, 25]. This view is well supported by a great 

 number of experiments, involving operative separations and transplanta- 

 tions, or transformations obtained by chemical means as those described 

 above, cf. [9, 15, 21]. 



The main point of attack of the controlling agents is probably the 

 nucleus and particularly the synthesis of ribonucleic acid within the 

 nucleus [18, 25]. 



So far chemical changes of the ribonucleic acids have not been directly 

 demonstrated during the development of the sea urchin embryo, cf. [5]. 

 It is of some interest that by certain cytochemical tests differences between 

 the nuclear ribonucleic acids of the animal and those of the vegetal em- 

 bryonic region could be demonstrated. The tendency for "unmasking" of 

 the phosphate groups of the ribonucleic acid seems to be greater in the 

 former than in the latter, cf. [10, 17] and a recent review [25]. These 

 differences become obvious only in the stage in which the primary mesen- 

 chyme begins to immigrate. The gastrulation initiates more direct inter- 

 actions between the germ layers. This holds not only for the material so 

 far considered — the sea urchin embryo — but also for amphibia and 

 vertebrates in general. I have, however, to refrain from details. 



In the progress of differentiation mechanisms arise that stabilize the 

 attained differentiations. These mechanisms may act by repressions to 

 some extent analogous to those found in bacterial systems, cf. [11, 20]. As 

 well known, one has here been able to distinguish two kinds of genes, the 

 structural gene and the regulating gene, the latter operating bv production 

 of repressor. Just as "structural" genes underlying for example the 

 animal and vegetal pathways are activated in the course of early embryonic 

 development, regulating genes may also gradually be activated. The 

 repressors produced may act at the cytoplasmic or the nuclear level.* The 

 latter kind of repression may possibly be realized in the work of Briggs and 

 King, cf. [4], dealing with transplantations of nuclei from embryonic 

 nuclei of frog into enucleated egg cells. Such nuclei are able to promote 



* This may be the mechanism of "canalization" in the sense of Waddington 

 [29]. 



