DIFFERENTIAL DEVELOPMENTAL MODIFICATION. II 219 



ing action are, in general, similar in type to the differential recoveries but 

 are usually, as might be expected, less extreme and with a degree of sec- 

 ondary modification differing with different agents. For example, second- 

 ary modifications are slight in cyanide; but in various other agents, even 

 in LiCl, they may be considerable or even extreme. A characteristic form 

 developing in 90 per cent sea water is shown in Figure 88, H; the preoral 

 region is relatively very large and almost spherical, the postoral region 

 and entoderm are still small. Forms of this type develop from gastrulae 

 with slight apical inhibition. In low concentrations of neutral red and 

 Nile blue sulphate similar forms develop. Figure 88, /, is an example from 

 a Nile blue sulphate lot in which the dye gradually disappeared, both 

 from the solution (open to the air) and from the animals. Evidently there 

 is a differential in ability to dispose of the dye in some way. Entoderm 

 stains more deeply than ectoderm and may remain differentially inhibited, 

 as in the figure, or show some secondary modification apically. 



Of the two starfish coeloms, originating as localized budlike outgrowths 

 on right and left sides of the foregut and separating from it as small 

 vesicles, the left is usually the larger, or becomes larger, gives rise to the 

 madreporic canal, and becomes the hydrocoel. Reversal or obliteration 

 of this asymmetry, as an occasional occurrence under natural conditions, 

 has been noted repeatedly and induced experimentally; and single coe- 

 loms, either dorsal or ventral, have also been observed in sea urchins 

 under experimental conditions.'^ These modifications of coelom develop- 

 ment occur in Patiria under all inhibiting conditions thus far used by the 

 writer. They are undoubtedly to be regarded as cases of differential in- 

 hibition. When the enlarged apical region of the archenteron extends ven- 

 trally toward the stomodeal region, growth in length is much greater on 

 the apicodorsal side than on the other. This is evident in the form of the 

 foregut in later stages (Fig. 83, A). With differential inhibition of this 

 development the coeloms tend to appear nearer the median line on the 

 dorsal side. All degrees of approximation to the median region appear 

 from a slender connection between the two (Fig. 89, A) to development 

 of a single median sac, bilaterally symmetrical with two madreporic canals 

 (Fig. 89, B) or with a single median dorsal canal (Fig. 89, C). With con- 

 tinuous exposure to inhibiting conditions the differential elongation of 

 the foregut may be completely inhibited so that it remains a blind, 

 rounded, or somewhat elongated region at the apical end of the archen- 

 teron. If coelomic development occurs in such cases^ it is usually apical 



"> Newman, 1925, Patiria, low temperature; see also Newman, 1921a, b, 1922; Runnstrom, 

 19256, c, Psammechinus, hypotonic sea water. 



