EXPERIMENTAL EMBRYOLOGY 159 



tioii process is carried out b}^ this alternating series of dynamic processes 

 and material reactions, the former involving the egg system as a whole 

 and enabling the reactions of more or less delimited substances and struc- 

 tures, the centers, to take place. In this light, regulation is not depend- 

 ent upon the powers of definite centers but upon dynamic processes or 

 structures made possible through such processes. It foUow^s that regu- 

 lative ability is limited by the degree of rigidity of the arrangement of all 

 the substances and structures necessary for development. Regulation 

 can occur only in that part of the egg in which normal dynamic processes 

 can happen, and the degree of regulation depends upon the extent to 

 which they can proceed unhampered. As Seidel says, the entire egg 

 nust be regarded as a system in which not only the embryonic tissues and 

 included factors but also the extraembryonic parts must be held respon- 

 sible for the determination of the organ regions. 



In no other group is there a set of data for any one species sufficiently 

 extensive to permit such a complete analysis of factor interaction. 

 Reith (1931, 1932, 1935) shows an activation center at the posterior end 

 of the egg of the ant and the weevil. The action of its product is similar 

 to that in the damsel fly, but the onset of its action is not dependent on 

 the entrance of cleavage nuclei into the region but is initiated presumably 

 by some part or product of fertilization. However, in the beetle Bruchus 

 Brauer and Taylor (1934) give a brief report indicating that the cleavage 

 nuclei are necessary for the action of the activation center. Schnetter 

 (19346) was unable to demonstrate an activation center in the honeybee 

 egg, but he has shown the presence of a differentiation center which 

 seems to bear a causal relation to the development of the embryonic 

 rudiment. These data, although differing in certain details from the 

 damsel fly, indicate that dynamic factors are responsible for the arrange- 

 ment of the structural elements, since regulation involves a shift in the 

 site of initial differentiation along w^ith that of the potency regions. The 

 more rigid the system the less the regulative power — a principle that 

 may involve the viscosity of the cytoplasm as suggested by the results 

 of centrifuging the eggs of other insects. In these insects, also, determi- 

 nation is probably brought about by a harmonious series of interacting 

 processes. 



In the determinate type of egg (Reith, 1925; Pauli, 1927; Sturtevant, 

 1929) we must assume that the determination attained by such series of 

 events is in large measure completed by the time of fertilization so that 

 the various egg regions are "self-differentiating." 



THE ANLAGEN PLAN OF THE EMBRYO 



The only satisfactory worked-out anlagen plan of any indeterminate 

 or incompletely determinate type of insect egg is that given by Seidel 



