226 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



inhibitions, all or almost all of the exogastrular entoderm may develop 

 from the evaginated original entoderm (Fig. 90, A-C). With the less ex- 

 treme inhibitions three entodermal regions may be more or less distinctly 

 marked off by constrictions (Fig. 90, J5, C). In recovery after greater in- 



FiG. 92, A-I. — Echinoid exogastrulae with small entoderms, resulting from exposure to 

 LiCl beginning in later pregastrular stages or at beginning gastrulation. A-C, Detidraster; 

 D-G, Strongylocentrotus purpuratus; H, I, S. Jranciscanus. A, B, m/45, 39 hr. from 6-hr. 

 blastula; C, m/25, 24 hr. from beginning gastrulation; D-G, m/40, 20 hr. from late blastula; 

 H, I, m/40, 24 hr. or more from late blastula. Arrows in G and // indicate directions of prog- 

 ress of dye reduction in low oxygen and numerals i and /, that dye reduction becomes evident 

 at about the same time in the regions indicated {D-I from Child, 19366). 



hibition development of two entodermal regions of large size is common, 

 often with a third region of small size or barely indicated (Fig. 91, A-C, 

 E-G). After extreme inhibition there is little or no regional differentiation 



(Fig. 91,^)- 



When exposure to lithium begins in late pregastrular stages or, in mod- 

 erate concentration, continues up to, or through, gastrulation, entoderm 



