DIFFERENTIAL DEVELOPMENTAL MODIFICATION. II 213 



derm, which is relatively or absolutely "too large." In the first gastrula 

 of Figure 84, 24 hours in water after LiCl, the thin apical ectoderm and 

 the extremely large apical entoderm suggest some degree of differential 

 recovery. 



As invagination progresses, entoderm becomes more susceptible than 

 ectoderm, at least in its more apical levels, and when sufficiently in- 

 hibited it begins to lose its epithelial character and to undergo dissocia- 

 tion. The lesser degrees of this change resemble immigration of mesen- 

 chyme in echinoids, but dissociation may involve the whole entoderm 



Fig. 84, A-E. — Differentially inhibited gastrulae of Patiria. A, LiCl m/25, 11 hr. from 

 sixty-four cells, 24 hr. water; B, LiCl m/30, 14 hr. from sixty-four cells, 26 hr. water; C, 

 LiCl m/2S, 195 hr. from thirty-two cells, 4 days water; D, LiCl m/25, n hr. from sixty-four 

 cells, 24 hr. water; E, KCN m/200,000 from two cells; perhaps some differential recovery in 

 apical archenteron of A and B. 



(Fig. 85). In the case of Figure 85, A, exposure to a very high concentra- 

 tion of LiCl (m/io) began just before gastrulation ; some invagination 

 occurred before the agent became fully effective, but gradually the apical 

 entoderm began to lose epithelial order and to dissociate, and the change 

 progressed basipetally but the ectoderm remained intact. This was the 

 characteristic effect in the experimental lot. In Figure 85, B, from another 

 lithium lot, the invaginated entoderm has become a solid cell mass and is 

 dissociating. These results, characteristic of lithium in sufficient concen- 

 tration, with exposure beginning just before gastrulation (A) or continu- 

 ing into that stage, indicate the change in condition of the entoderm 

 associated with gastrulation. As in echinoids, lithium in early pregastru- 



