252 PATTERNS AND PROBLEMS OF DEVELOPMENT 



region, the dorsal region next; the developmental modifications show 

 them to be the most susceptible regions. 



In uninhibited individuals the sensory vesicle contining the two pig- 

 mented sense organs lies somewhat to the right of the median dorsal 

 region. Among the more inhibited forms, individuals apparently com- 

 pletely bilateral, with pigment spots bilaterally localized at some distance 



Fig. ioo, .4-G.— More extreme differential inhibitions of Corella; in all the most advanced 

 stage of tail development is figured; all show dorsiventral differential inhibition (from Child, 

 1927 d). 



from the median plane, appear rather frequently. The pigment spots are 

 sometimes connected by a band of pigment like the eyespots of tera- 

 tophthalmic planarian heads (Fig. loi). It would be of interest to know 

 what sort of ascidian would develop from these apparently completely 

 bilateral forms, but in all observed thus far there has been little or no 

 development beyond the stages figured. 



In uninhibited development the larval tail shows a very distinct gradi- 

 ent with high end at the tip. At or before the beginning of tail resorption 

 this gradient disappears (pp. 145-47)- With differential inhibition the 

 caudal gradient is decreased or quite obliterated; consequently, the tail 

 is smaller, shorter, or a mere cell mass containing chorda cells but without 



