514 PATTERNS AND PROBLEMS OF DEVELOPMENT 



is more nearly a primary factor in developmental pattern than specific 

 substance differences. 



Between the data of embryonic sea-urchin reconstitution and those in- 

 dicating existence of a primary, and later of a secondary, activity gra- 

 dient there is no conflict. Different substances may be present in apical 

 and basal regions or be formed as development progresses; and as far as 

 they are present, they are doubtless factors in differentiation of ectoderm 

 and entoderm, but entodermization and ectodermization in consequence 

 of isolation and combination of parts and of differential susceptibility to 

 chemical and physical agents appear to be more directly related, at least 

 as regards their initiation, to nonspecific differences in metabolic activity 

 than to specific substances. 



In the echinoids for which data are available the secondary basal re- 

 gion of activation becomes more effective as a dominant region and appar- 

 ently as an inductor than the apical region. This accords with the fact 

 that dye reduction and susceptibihty suggest that it represents a more in- 

 tense activation than any other part of the embryo at the stages con- 

 cerned. Also, entodermization of presumptive ectoderm seems to occur 

 more frequently in Horstadius' experiments than ectodermization of pre- 

 sumptive entoderm. Probably future experiment will show differences in 

 different echinoid species in activation of the basal region. No data of 

 differential dye reduction have been obtained for Arbacia punctulata, 

 and the capacities of the micromeres as inductors are not known, but the 

 characteristics of gastrulation suggest that basal activation may be less 

 intense in that species than in some others. It seems to be less in the 

 asteroid Patiria, preceding invagination, than in echinoids (p. 134). 



RECONSTITUTION IN INSECT DEVELOPMENT 



Investigation of reconstitutional potencies in eggs and embryos of most 

 arthropods is, to some extent, limited by the character of the chorion and 

 the impossibility of removing the earlier stages from the egg envelope. 

 However, by means of Hgature at different levels, localized cautery, and 

 ultra-violet radiation it has been possible to obtain some information 

 concerning potencies of early developmental stages and the progress of 

 physiological differentiation. Earlier experiments on chrysomehd beetles 

 indicated absence of reconstitution at the developmental stages concerned 

 (Hegner, 1908, 191 1). Results of ligaturing and microcautery of early 

 stages of fly development also indicated that at the earliest stages avail- 

 able after eggs were laid reconstitution did not take place (Reith, 1925; 



