DIFFERENTIAL DEVELOPMENTAL MODIFICATION. II 231 



increase in susceptibility and apparent activation of entoderm (pp. 

 134-39). Exogastrulae of the starfish Patiria (Fig. 95) show various de- 



I 



Fig. 95, A-I. — Lithium exogastrulae of Patiria. A-F, continuous exposure from two- 

 cell stage: A, m/30, 60 hr.; B, m/40, 77 hr.; C, D, m/40, 95 hr.; E, F, m/40, 94 hr., slight 

 secondary modification apically. G-I, differential recoveries: G, H, m/30, 18 hr. from first 

 cleavage, 2 days water; /, m/20, 19 hr. from first cleavage, 3 days water; H and /, e.xentogas- 

 trulae. Arrows indicate directions of progress of dye reduction in low oxygen, and numerals 

 I and 2 the time-order in which reduction becomes evident in different regions (from Child, 

 19366). 



grees of entodermization of prospective ectoderm; cells often dissociate 

 from entoderm into the blastocoel, resembling the primary mesenchyme of 

 echinoids (Fig. 95, A-C, E, G); and the apical ectoderm, when strongly 



