DIFFERENTIAL DEVELOPMENTAL MODIFICATION. II 223 



reduction in relation to exogastrulation suggested an interpretation of this 

 series of developmental modifications, differing somewhat from hypothe- 

 ses advanced by others (Child, 19366). Further experiment, chiefly with 

 Dendr aster as material, appears to throw some additional light on condi- 

 tions concerned in development of these extremely interesting forms. 



ECHINOID EXOGASTRULAE: A FEW EXAMPLES OF FINAL STAGES 



Developmental modifications of exogastrulae are essentially similar in 

 the echinoid species thus far used as material. The following account is 

 chiefly concerned with lithium exogastrulae of Dendr aster excentricus. Fig- 

 ure 73, A-E, showing forms of gastrula and later stages under natural con- 

 ditions, will serve for comparison with exogastrulae. A few of the almost 

 innumerable modifications of form and proportion in exogastrulae are out- 

 lined in Figures 90 and 91," most of them lithium forms developing with 

 different concentrations and exposure periods, beginning at, or soon after, 

 the first cleavage. Attention is called first to the differential inhibitions 

 of the ectoderm in these forms. With the lower concentrations or rela- 

 tively short exposures ectoderm may attain more or less completely 

 pluteus form (Fig. 90, A, B). Figure 90, C, a result of crowding, shows 

 somewhat less advanced ectodermal development. With greater inhibi- 

 tion ectoderm is smaller and oral lobe is almost completely inhibited, but 

 arms may develop at varying wide angles up to 180° (Fig. 90, D-F). As 

 will appear more clearly below, the basal part of the prospective ectoderm 

 of these forms has been entodermized by lithium, that is, has become 

 entoderm; consequently, the arms develop from ectoderm much farther 

 apical than the original arm area. These forms are similar as regards 

 ectodermal development to the inhibited, wide-angled modifications dis- 

 cussed above (pp. 200-201) and undoubtedly result from essentially similar 

 conditions. They appear in a wide range of concentrations according to 

 exposure period. More inhibited ectoderms develop parallel, fused, or 

 single arms with double- or single-arm skeleton, and in some of them a 

 stomodeum develops in a position relative to the arm, which indicates 

 that the arm is ventral and median (Fig. 91, yl), as in the one-armed forms 



" These and following figures concerned with exogastrulation in Dendraster are drawn to 

 the same scale from ocular-micrometer measurements of living individuals and sketches of 

 the same individuals made at the same time. Levels of localization of mesenchyme are indi- 

 cated; but, as regards actual numbers of cells of mesenchyme or dissociated entoderm in the 

 blastocoel, the figures are diagrammatic, though they attempt to show whether free cells are 

 numerous or few and to represent loss of epithelial character and dissociation in entoderm. 

 Pigment cells are indicated, chiefly along the ciliated band, where they are most numerous 

 when it develops. Patiria exogastrulae in Fig. 95 are on a smaller scale. 



