212 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



rounded preoral region, empty except for mesenchyme; fourth, two ciH- 

 ated bands extend obhquely around the body, as indicated in Figure 83. 



DIFFERENTIAL INHIBITION 



Experiment on asteroids is less extensive than on echinoids, but a num- 

 ber of agents have been used to produce modifications.^ Not the sHghtest 

 evidence has appeared in more than a hundred experimental lots of spe- 

 cific relation between any of these agents and any particular modification, 

 not even between lithium, exogastrulation, and entodermization of pro- 



FiG. 83.— Lateral and ventral outlines of larva of Paliria miniata; ciliated bands indicated 

 by broken lines (from Child, 1938). 



spective ectoderm. Eggs from animals kept in laboratory aquaria sup- 

 plied with flowing sea water for 9 or 10 days before use develop differen- 

 tially inhibited forms identical with those obtained with cyanide and 

 lithium, etc. With exposure to the agent in inhibiting concentrations at 

 beginning or early cleavage inhibition at first decreases basipetally, as in 

 echinoids. In differentially inhibited blastulae the polar axis is short and 

 the apical ectoderm thicker than normal. If the inhibition is not too ex- 

 treme to permit gastrulation, these blastulae, returned to water shortly 

 before gastrulation, or gastrulating before return, form gastrulae like 

 those of Figure 84. In all of these ectoderm is more inhibited than ento- 



5 The following agents have been used: LiCl and KCN in many concentrations; hypotonic 

 sea water; the dyes neutral red, Janus green, and Nile blue sulphate; crowded conditions; and 

 sea water at pH 7, with CO, probably the cflective agent. 



