156 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



point irradiation before or during the time of cell aggregation, result in 

 changes in the length, shape, and position of the embryonic rudiment. 

 The spaces between the yolk and chorion thus formed, or resulting when 

 a ligature is loosened, serve as foci for cell aggregation which can thus be 

 artificially and prematurely produced outside the normal region. These 

 data indicate that the direct action of the differentiation center is due to 

 a wave of contraction in the yolk system spreading anteriorly and 

 posteriorly from the center, the yolk system retracting from the chorion 

 in such a way that the evenly distributed blastoderm cells are forced to 

 fill the resulting space. In this manner cells first aggregate in the region 

 of the differentiation center, and the size and shape of the germ band are 

 molded by the space between yolk and chorion. This makes it clear how 

 the differentiation center can be the center of a field of activity without 

 necessarily involving the actual transport of any material substance, the 

 result of constrictions being due to interference with yolk movements. 

 Additional proof of the dynamic nature of the differentiation processes in 

 Platycnemis is furnished by the fact that killing a girdle of cells by ultra- 

 violet irradiation either in front of or behind this region where visible 

 differentiation normally begins causes a corresponding shift (backward or 

 forward, respectively) of the position of the initial differentiation. The 

 yolk system is, in effect, temporarily reduced in size, and the visible 

 differentiation begins in the same relative position within the new system 

 that it held in the larger system. If, then, the differentiation center is a 

 definite region within the egg (as it seems to be), it must remain at one 

 end of the reduced system while its action originates in a new location. 



In the camel cricket (Tachycines) (Krause, 1934) the order of decreas- 

 ing regulation ability (lost earliest by the thorax), the relative frequency 

 of organ duplications (least often in prothorax), and certain asymmetrical 

 duplications show that differentiation along both longitudinal and trans- 

 verse axes proceeds anteriorly and posteriorly from the first thoracic 

 segment. He suggests that the presence of a differentiation center may 

 account for the nonoccurrence of doubling along the longitudinal axis. 



The results of constricting eggs at successive points along their 

 length enabled Schnetter (19346) to mark off "potenc}^ regions" in the 

 12-hour blastoderm of the honeybee egg. Each of these regions when 

 present in whole or in part guarantees the complete formation of the 

 corresponding embryonic systems (cf. "harmonic equipotential regions," 

 e.g., hmb field in Amphibia). Thus regulation in the honeybee egg is, 

 in general, "stepwise," a given region developing as a whole or not at all. 

 In Fig. 60a it can be seen that all the potency regions lie within or extend 

 into the region of the differentiation center. Accordingly it may be 

 regarded as a "concentration center" for potencies enabling whole forma- 

 tion of the various organ regions. Figure 606 shows that there is a shift 



