220 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



and may show some bilaterality (Fig. 89, E) or be single and median 

 (Fig. 89, F). In some cases when the foregut elongates, a single coelom 

 develops on the ventral side (Fig. 89, G, H), between it and the stomodeal 

 region. The various degrees of differential modification from bilateral to 

 median origin of the coeloms with approximation, partial fusion, or single 

 median coelom with bilaterality obliterated present a series of differential 



Fig. 89, A-H. — Modifications of coelom development by differential inhibition in Patiria; 

 outline of foregut and part of midgut with coelom: A, B, D-F, from dorsal side, C, G, H, from 

 left side. A, right and left connected, asymmetry evident; B, single symmetrical and median 

 with two canals, both 1 1 hr. LiCl m/25 from thirty- two cells; C, single median dorsal with medi- 

 an dorsal canal, LiCl m/40, continuous from two-cell stage; D, single symmetrical without 

 canal, LiCl m/30, 2ii hr. from two-cell stage; E, F, apical (anterior), LiCl m/40, continuous 

 from two-cell stage; G, single median ventral, between foregut and stomodeum, eggs from 

 animals long time in laboratory; H, single median ventral, LiCl m/30, 38I hr. from si.xty-four 

 cells, undoubtedly some recovery but occurring late, after ventrodorsality of foregut appar- 

 ently reversed. 



inhibitions similar to those of the planarian head, the anal arms of the 

 sea-urchin larva, and the approximation to the median line of eyes and 

 other bilateral organs of the vertebrate head. In all these cases the modi- 

 fications evidently depend on a mediolateral physiological differential, a 

 median region being more inhibited than lateral; and as inhibition in- 

 creases, lateral organs appear successively nearer the median line and 

 become median. Coelom development on the ventral side of the foregut 

 probably results from reversal of the ventrodorsal differential by inhibi- 



