154 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



constrictions, yet obviously they cannot be the same substance. Another 

 interesting difference from Platycnemis is that the activation center in 

 this species induces a visible zonation (differential thickening) of the 

 cortical layer prior to the migration of the cleavage nuclei. This visible 

 differentiation begins at the posterior pole and passes slowly to the 

 anterior end of the egg, as does also the ''activating power." Attempt- 

 ing to localize the activation center to a visibly distinct part of the 

 posterior region of the egg, he noted that the disintegration of the "pole 

 disk" is correlated with the action of this center. The pole disk, how- 

 ever, cannot be the essence of the center, since it is lacking in the ant 

 Lasius which gives similar experimental results. Nor can the effect be 

 traced to the bacterial symbionts, since their injury is not correlated with 

 that of the embryo. As in Platycnemis, the activation center must be 

 ascribed to an invisible difference in the egg regions. 



In the beetles Bruchus and Sitona Brauer and Taylor (1934) and 

 Reith (1935) report the presence of an activation center in the posterior 

 region of the egg. In both these beetles the functioning of this center 

 begins during cleavage and is finished when the nuclei enter the cortical 

 layer {i.e., later than in ants). In Bruchus, Brauer and Taylor state that 

 when the constriction is made sufficiently earl}^ to exclude nuclei from 

 the posterior end of the egg, the anterior por.tion forms "only a poorly 

 developed blastoderm," yet "a protoplasmic isthmus . . . however 

 narrow it may be, serves to conduct the organizing principle anteriorly." 

 Hence it would seem that the nuclei are necessary for the functioning of 

 the activation center in Bruchus as in Platycnemis. 



The Dififerentiation Center. — The visible differentiation of the insect 

 embryo begins in the presumptive prothorax and from this region 

 proceeds anteriorly and posteriorly. Descriptive embryology shows that 

 the spread of differentiation from the thoracic region is a general prin- 

 ciple of insect development and, as Schnetter (1934a) points out, may be 

 considered as typical of insects as is superficial cleavage, though there 

 may be exceptions in regard to both (in other arthropods and annelids 

 differentiation begins in the head). The term "differentiation center" 

 is here restricted to a physiological concept and is not applied to the 

 visible starting point of differentiation, even though physiological and 

 morphological phenomena normally occurring at a common locus may 

 at times appear to be experimentally separable. 



In Platycnemis the region where the two rows of assembled nuclei first 

 come together to form the germ band marks the place from which all 

 later differentiation proceeds anteriorly and posteriorly ; it lies in the pre- 

 sumptive thoracic region, about one-third the length of the egg from the 

 posterior pole (Seidel, 1926, 19296, 1934). In the honeybee (Schnetter, 

 1934a) the visible embryonic differentiation begins near the anterior end 



