EMBRYONIC RECONSTITUTIONS 517 



Bildungszentrum at the posterior end of the egg is to determine a "differ- 

 entiation center," corresponding in position to the pro thoracic segment. 

 The aggregation and more rapid division of nuclei begins there, and de- 

 velopment proceeds both anteriorly and posteriorly from this region. 

 Seidel (1934) regards the differentiation center as in some way an organ- 

 izer, but not by means of a substance, since incomplete ligatures block its 

 action. With incomplete hgature anterior to this center in early blasto- 

 derm stages no embryo formation occurs anterior to the ligature (Fig. 165, 

 G) ; but with ligature in the differentiation center embryo formation may 

 occur both anterior and posterior to it (Fig. 165, H), and with hgature 

 posterior to the center an embryo forms only anterior to the ligature 

 (Fig. 165, /). 



With other insects as material other investigators have found that pos- 

 terior ligature or cautery in very early stages prevents embryo formation 

 but at later stages may be followed by normal or partial development, 

 according to species.** As regards the differentiation center, however, there 

 is not complete agreement. According to Keith, the head region is the 

 differentiation center in Camponotus, but the evidence concerning the 

 beetle Sitona is inconclusive. He suggests that the differentiation center 

 is the region of "greatest physiological activity." The embryo of the honey- 

 bee, which occupies the whole length of the egg, shows very considerable 

 reconstitutional potency, even 24 hours after laying. Dwarf complete em- 

 bryos may develop posterior to a ligature separating the anterior fifth of 

 the egg, but there is usually no development anterior to the hgature. 

 With ligatures further posterior there may be partial development both 

 anterior and posterior. Seidel's hypothesis of a prothoracic differentiation 

 center is regarded as supported by these experiments (Schnetter, 1934a, h; 



1936). 



Extensive duplications of any or all parts have been produced in an 

 orthopteran, Tachycines asynamoriis, by local injuries and by separation, 

 by glass needles, of parts of the small embryonic primordium situated at 

 the extreme posterior end of the egg; and presence of a thoracic differentia- 

 tion center is inferred, chiefly, it appears, from a single case of duplication 

 (Krause, 1934). In a later paper Krause (1938) finds the high point in 

 cleavage and sequence of differentiation at the level of the cephalic lobes; 

 evidence for a differentiation center in this orthopteran appears only in 

 the space-time order of differentiation posterior to the head. 



Early developmental stages of Drosophila subjected to certain ranges 

 8 Reith, 1931, an ant, Camponotus ligniperda; 1935, a beetle, Sitona lineata. 



