520 PATTERNS AND PROBLEMS OF DEVELOPMENT 



interrupted, though in the Hgature experiments in relation to the differ- 

 entiation center at somewhat later stages the susceptibility of the cortex 

 may have increased to such an extent that incomplete ligature is sufScient 

 to produce a stimulus decreasing or obliterating the gradient of more an- 

 terior regions. The effect of cautery should be essentially similar to that of 

 section of the cortex by ligature. The insect egg is apparently highly sensi- 

 tive. Seidel observed contraction of the yolk about cauterized regions, and 

 Reith (1925) saw a wavelike contraction {Zucken) pass over the egg of 

 Musca when it was cauterized locally. The assumption that a part of the 

 insect egg can be pinched off by ligature or killed by burning without ac- 

 tivation or excitation extending to other parts seems, at present, unwar- 

 ranted, in view of the data on postembryonic reconstitutions and some 

 other embryonic reconstitutions. With complete obliteration of the an- 

 teroposterior gradient by an opposed gradient the embryonic region must 

 become apolar, as it actually does after complete ligature and cautery in 

 early stages. It gives rise to a nonembryonic blastoderm without differen- 

 tiation. In early stages the activation or excitation resulting from poste- 

 rior hgature or cautery may be sufhcient to obliterate the whole embryonic 

 gradient, or its effect may be partial and involve only abdominal, or ab- 

 dominal and thoracic, regions (Brauer and Taylor, 1936). In Tenebrio a 

 head, or only the anterior part of a head, may develop after posterior 

 cautery (Ewert, 1937). Later, after the anteroposterior gradient and de- 

 termination along its course have progressed farther and become more 

 stable, posterior ligature or cautery or ligature has little or no effect. It 

 may also be questioned whether inhibition of development anterior to a 

 ligature just anterior to the differentiation center is due to isolation from 

 the center or to obhteration of the gradient in the anterior region by the 

 opposed gradient resulting even from incomplete ligature in the more 

 susceptible anterior region. In short, the experimental data suggest that 

 inhibition of development anterior to a region of ligature or cautery may 

 be due, not to isolation from a center, but to obliteration of an antero- 

 posterior gradient by an opposed activation or stimulus resulting from the 

 injury. Absence of development posterior to a ligature in early stages is 

 doubtless due to isolation from a dominant region. These suggestions are 

 in line with results of postembryonic reconstitutions in lower inverte- 

 brates. Section a short distance proximal to a hydranth primordium in 

 early stages may inhibit it completely, and posterior section within a cer- 

 tain distance of the anterior end of a piece inhibits planarian head develop- 

 ment. No Bildungszentrum is involved in these cases. Apparently there is 



