206 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



region. Comparison of the Arhacia plutei of Figure 77, D-G, with those 

 developing under natural conditions (Fig. 73, B, E) shows the alteration 

 in proportions. Similar modifications with low concentrations of LiCl ap- 

 pear in Dendraster: Figure 77, H, is an example of differential tolerance or 

 conditioning to lithium, and / and / of Figure 77 are cases of differential 

 recovery from lithium: pluteus form under natural conditions is shown 

 in Figure 73, F. In all these wide-angled plutei the ciliated band has the 

 usual relation to oral lobe and arms, and the characteristic skeletal pat- 

 tern develops but with alterations in proportions corresponding to those 

 of other parts. 



Fig. 78, A-E. — More extreme secondary modifications in Arhacia with conditioning to, 

 and recovery from, alcohol, acid sea water, CuSO^, HgCL, etc. A-C, ventral, anal, and lateral 

 outlines of an individual; D, E, ventral and lateral outlines of another individual without 

 skeleton (after Child, I9i6fc). 



With somewhat greater degrees of inhibition the secondary modifica- 

 tions are greater. Characteristic forms of Arhacia are outlined in Fig- 

 ure 78. These larvae may be said to consist largely of oral lobe; this is 

 much enlarged and elongated relatively to other parts but still shows 

 inhibition in later differentiation. The larvae are flattened ventrodorsally, 

 and arms are more or less inhibited or absent. The skeleton may be lim- 

 ited to a single rod, as in Figure 78, A-C, or the brachial rods of the two 

 sides may be separate; and in some animals there are other spicules or 

 the skeleton may be entirely absent (Fig. 78, D, E). Here the secondary 

 modifications of ectoderm are evidently apicoventral and decrease basip- 

 etally. The usually large foregut suggests secondary modification in the 

 apical entodermal region, the high end of the entodermal gradient. The 

 skeleton is more or less inhibited or absent, probably because activation 



