2o8 PATTERNS AND PROBLEMS OF DEVELOPMENT 



secondary modification ventrally {D). In somewhat more inhibited indi- 

 viduals secondary modification may be limited to the apical region. In 

 Figure 79, £, F, and G, relatively extreme examples are shown of a type 

 very common with certain ranges of concentration and exposure for all 

 agents used. Ventrodorsality is obliterated in the body, and a transverse 

 ciliated band surrounds the basal region. •^ This altered position of the 

 band is characteristic of these forms and also appears in many exogastrulae 

 (pp. 224-25). Apparently the more apical levels of the ventral side are 

 so much inhibited that the band cannot develop there; but with secondary 

 modification in the less inhibited basal region and the altered relations of 

 regions it differentiates there, in part with localization different from the 

 normal. However, the tip of the secondary apical outgrowth often de- 

 velops the specialized ciliated epithelium characteristic of the band. 



Inhibition beginning just before, or at the earliest stages of, entodermal 

 invagination and within the range permitting some degree of secondary 

 modification gives results of interest. At these stages entoderm and mes- 

 enchyme have undergone or are undergoing increase in susceptibility and 

 rate of dye reduction. Conditioning or recovery occurs in the apical re- 

 gion, and entoderm and mesenchyme are more or less inhibited. These 

 modifications appear with the various agents used, even in a certain range 

 of concentrations of LiCl (Fig. 80). In earlier stages lithium may ento- 

 dermize prospective ectoderm, as already noted, and so increase entoderm 

 at the expense of ectoderm; but at this stage entoderm has become so 

 highly susceptible that it is inhibited, while the apical ectoderm, at first 

 inhibited, gradually becomes thicker and elongates. The lithium forms 

 of Figure 80 do not usually develop much farther with continued exposure. 

 No skeleton is formed, the mesenchyme, not shown in Figure 80, remain- 

 ing scattered or irregularly massed in the blastocoel. Incidentally it may 

 be noted that these forms with relative enlargement of the apical region 

 suggest an approach to asteroid larval form with its large preoral region 

 (Child, 1938). The apicobasal dye-reduction gradient in early stages of 

 the starfish appears to be steeper than that in the sea urchin (Child, 

 1936a), and these differential modifications involve an increase in steep- 

 ness of the gradient, at least at the more apical levels. 



Lesser degrees of differential tolerance or conditioning are outlined in 

 Figure 81, and of differential recovery after temporary exposure in 



3 In some individuals the band develops as two separated parts symmetrically localized on 

 the two sides, indicating that the bilateral features of ventrodorsality have not been completely 

 obliterated. 



