5i8 PATTERNS AND PROBLEMS OF DEVELOPMENT 



of intensity and duration of ultra-violet radiation show no defects in em- 

 bryonic or larval stages, but teratological modifications and defects ap- 

 pear in the imago (Geigy, 1931). Radiation of different early stages shows, 

 according to this author, a sensitization progressing ''in the craniocaudal 

 or, better, thoraco-abdominal" direction, followed by a densensitization 

 progressing in the same direction. Modifications of the imaginal head 

 were not obtained in any case. These experimental results lead the author 

 to accept the hypothesis of a prothoracic differentiation center. 



In the lepidopteran Ephestia posterior cautery does not prevent axiate 

 development, though it may produce defects of posterior regions. Cell 

 formation, as nuclei reach the egg surface, progresses anteriorly and pos- 

 teriorly from the differentiation center (Maschlanka, 1938). 



Experiments with a beetle, Bruchus quadrimaculatus , show prevention 

 of axiate development by posterior constriction or cautery in early stages, 

 with decreasing effect as development progresses. The differentiation cen- 

 ter is maxillar-prothoracic (Brauer and Taylor, 1936). In further experi- 

 ments with the same species on developmental modification in relation to 

 susceptibihty to KCN, differential inhibitions of development have been 

 obtained. With certain periods of exposure to cyanide in early stages, the 

 median ventral maxillar-prothoracic region shows the greatest injury with 

 decrease anteriorly, posteriorly, and laterally from the anteroventral posi- 

 tion of this region (Fig. 166, A-C). With inhibition of development, vari- 

 ous degrees of embryonic dupHcation may result, beginning at the level 

 of the differentiation center and extending anteriorly and posteriorly from 

 it, according to degree and extent of the differential inhibition, occa- 

 sionally resulting in complete duplication (Fig. 166, D, E). In these cases 

 the less susceptible lateral regions are physiologically isolated by inhibi- 

 tion of the median region and with recovery develop independently. At 

 stages when the embryonic plate is elongating, inhibition of the elongation 

 is the most conspicuous feature of modification by cyanide (Brauer, 1938). 



In embryos of the beetle Tenebrio molitor after posterior cautery in 

 early stages, heads, or even anterior parts of heads, may develop in com- 

 plete absence of all other parts (Ewert, 1937); the thoracic differentiation 

 center is not evident. 



All workers in this field agree, however, in finding that major reconsti- 

 tutions occur only in early developmental stages, if at all ; but, according to 

 Seidel's account, the pattern of early insect development is unique. The 

 hypothesis of a differentiation center activated or determined by another 

 center in the posterior region of the egg raises certain questions. First, as 



