2 so PATTERNS AND PROBLEMS OF DEVELOPMENT 



means physiological or chemical complexity, the hypothesis is merely an 

 opinion without basis of evidence. 



The only data on experimental differential modification of development 

 in arthropods are apparently those obtained by Brauer (1938) on em- 

 bryonic stages of a brucid beetle. In early stages (oviposition to 6^ hours) 

 axiate pattern can be completely obliterated by cyanide. In following 

 stages (6|-i2 hours) susceptibility decreases anteriorly, posteriorly, and 

 laterally from the presumptive prothoracic-maxillary region of the em- 

 bryonic plate. In consequence of complete inhibition of the median re- 

 gion, more or less complete duplication may result by formation of new 

 embryonic plates in the less susceptible and less inhibited lateral regions, 

 which would give rise to lateral parts under natural conditions. Partial 

 duplications of embryos show complete doubling in the prothoracic-maxil- 

 lary region with incomplete duplication of heads and more posterior parts; 

 complete dupUcations of all parts also occur (pp. 518-19). 



ASCIDIANS 



Very considerable differential modifications of development result from 

 action of external inhibiting factors on early developmental stages of the 

 ascidian Corella willmeriana (Child, 1927(f). Developmental stages under 

 natural conditions are outlined in Figure 99: the larva before hatching 

 {A), the fully developed swimming larva {B), a stage of tail resorption 

 (C), tail completely resorbed, with chorda cells aggregated in a mass {D). 

 Development to the swimming larva takes place in the atrial cavity of the 

 parent at a pH below that of normal sea water, and removal of eggs or 

 early embryonic stages from the atrial chamber to sea water inhibits de- 

 velopment differentially. Experiment indicates, however, that CO^ con- 

 tent of water, rather than hydrogen ion concentration, is the important 

 factor in the atrial chamber, the eggs apparently being conditioned to a 

 higher CO2 content than that of normal sea water. Sea water with alkahn- 

 ity increased by NaOH is more strongly inhibiting than natural sea water. 



The chief modifications thus far observed are indicated in Figures 99 

 (£-G), 100, and loi. In uninhibited larval development the tail always 

 coils ventrally around the larval body (Fig. 99, ^). With slight inhibition 

 it may be bent in various directions (Fig. 99, E), but with greater inhibi- 

 tion it extends dorsally instead of ventrally and is shortened and folded 

 or is represented by a rounded mass in which cells of the notochord are 

 visible but without definite order (Fig. 99, F, G, and Fig. 100). The dorsal 



