88 C. M. CHILD 



crinoid larva, for example, the blastopore closes completely, 

 the entoderm forms a closed vesicle in the blastocoel and the 

 anus breaks through at another point. All of these conditions, 

 even the new anus (fig. 67), appear in these recovery forms of 

 Arbacia. Likewise the basal ciliated ring recalls the ciliated 

 rings of the crinoid larva. The facts suggest that in the primi- 

 tive echinoderm larva, the apico-basal axial gradient is not as 

 well developed as in Arbacia, and the antero-posterior and bilat- 

 eral gradients are very slight, or become effective only in later 

 stages. By means of differential inhibition, which decreases 

 the slope of the apico-basal gradient and almost or quite oblit- 

 erates the minor gradients, we produce axial relations which re- 

 semble, in certain respects, those of the primitive forms in 

 which the axial gradients are less developed ; i.e., less permanently 

 recorded in the protoplasm and therefore less effective as fac- 

 tors in orderly development. 



Figures 74 to 81 show the lower limits of differential recovery. 

 In all, except perhaps figures 74 and 75, there has been some 

 apical loss. In figures 74 (A, anterior, B, lateral aspect) and in 

 figures 75 to 77, all recoveries after alcohol, the apical out- 

 growth develops the "ciliated band characteristic of the oral 

 lobe. In recovery after KCN, the ciliated band usually does not 

 differentiate where the degree of differential recovery is so slight. 

 Figures 78 to 81 show cases in which no oral lobe is differentiated, 

 but a mouth develops apically on an elongated apical region 

 (figs. 78, 79), or the body remains spherical and the develop- 

 ment of the mouth is the only evidence of differential recovery. 

 The development of the entoderm shows the same variations as 

 in figures 57 to 73. These forms are essentially similar to the 

 cases of differential acclimation in figures 49 to 53. 



The various degrees of differential recovery in figures 57 to 81 

 are the final stages of development attained. Such larvae may 

 live for a week or two and may show marked motor activity, 

 but they do not develop further. The only reason which can 

 be assigned for this failure to resume development, is incom- 

 plete recovery, and this means that the metabolic gradients 

 have been obliterated or levelled down to a greater or less degree 



