200 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



irreversible, at least up to the most advanced stage attained. Similar 

 forms appear in acid sea water, sea water plus ammonia, ethyl alcohol, 

 hypotonic sea water, MgClz, etc., with certain concentrations and exposure 

 periods. Unfertilized eggs in KCN, m/ioo up to a few hours, then fer- 

 tilized and developing in water, give similar forms. The forms of Figure 74 

 show not only an apicobasal but also a ventrodorsal and a mediolateral 



Fig. 75, A-I. — Differential inhibition in Arbacia, showing various degrees of obliteration 

 of the original ventrodorsaHty and the resulting bilaterality. A-D, ventrodorsality still 

 evident; E-H, bilaterality perhaps secondary and determined by transverse orientation and 

 elongation of skeletal rod; /, completely radial form without skeleton. Ciliated band shaded. 



differential inhibition. It seems evident that decrease of brachial angle, 

 approximation and fusion of arms, and development of a single median 

 arm result from different degrees of ventral inhibition, decreasing from 

 the median region laterally and quite similar in principle to the mediolat- 

 eral differential inhibition of planarian head development (pp. 177 81). 

 A series of forms with further degrees of differential inhibition and de- 

 crease of axiate pattern is shown in Figure 75. These forms occur most 

 frequently with long-time exposures (e.g., 36 hours or more) from early 



