158 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



In the higher Diptera the data from cauterization, constriction, and 

 centrifiigation (Reith, 1925; Pauli, 1927; Rostand, 1927; Strasburger, 

 1934) show that in all cases mosaic or partial embryos are produced by 

 elimination of an.y egg part irrespective of the age of the embrj^o. A 

 physiological differentiation center affecting larval organization is not 

 demonstrable, although Henshaw's data (1934) indicate that there is some 

 physiological regulator of early embryonic differentiation in Drosophila, 

 since only weak doses of X rays were required to block development 

 before gastrulation but much stronger doses were required later. 



Interaction of Centers and Other Regions of the Egg.— Seidel (1934) 

 has shown that in Platycnemis an interaction between the cleavage nuclei 

 and the region called the "activation center" results in a product that 

 diffuses forward in the egg (see Fig. 59a, 6). As this product passes 

 anteriorly, there is a visible change in the structure of the yolk. How- 

 ever, this product does not directly affect differentiation; it functions by 

 the activation of the differentiation center. The latter, in turn, appears 

 to induce the onset of heterochronous cell divisions and the contraction 

 of the yolk system which brings about the aggregation of cells to form 

 the germ band and the later shift in the position of the same. 



Comparison of the reactions of the activation center and differentia- 

 tion center of Platycnemis under similar restrictions indicates that they 

 differ in mode of action. A loose or temporary constriction in the 

 region of the activation center results in no abnormality but only delay. 

 But a loose or temporary constriction in the region of the differentiation 

 center (after functioning of the activation center) invariably results in 

 an abnormal embryo. This is elucidated by proof that the first functions 

 by the production and diffusion of a specific material substance, the 

 second by dynamic movement processes which must have some as yet 

 unknown physicochemical basis. 



The realization of the importance of dynamic phenomena in develop- 

 ment has enabled further analysis of the processes underlying determina- 

 tion and regulation and of the significance of the "centers." Such an 

 analysis, based primarily on his findings in Platycnemis, has been begun 

 by Seidel (1934). The predominant notes in his discussion are the impor- 

 tance of the entire egg as a substrate for dynamic processes in determi- 

 nation, the subordination of the so-called "centers" to the system as a 

 whole, and the alternation of dynamic processes and "material reactions" 

 during development. The latter is quite evident in the sequence of 

 events in Platycnemis, viz., the migration of cleavage nuclei, the reaction 

 between nuclear and cytoplasmic factors in the activation center, the 

 diffusion of the reaction products cephalad from the activation center, 

 reaction with the yolk system, and the contraction of the yolk system 

 originating in the region of the differentiation center. The determina- 



