CHAPTER VI 



DIFFERENTIAL MODIFICATION OF DEVELOPMENT: 

 ECHINODERMS 



BECAUSE of its plasticity under experimental conditions, echino- 

 derm development in its early stages is exceptionally interesting 

 material for investigation of differential modification of develop- 

 ment. The alterations of form and proportions and the changes in locali- 

 zation of parts resulting from exposure of the whole developing organism 

 to experimental environments are, to a high degree, definite in character 

 and experimentally reproducible. They are not simply "abnormal" or 

 teratological forms to be described as curious or mysterious anomalies of 

 development, but alterations in definite ways under controllable condi- 

 tions; as such, they constitute a highly significant body of evidence bear- 

 ing on problems of the physiology of developmental pattern. At present 

 little more than a beginning has been made in this field of physiological 

 analysis, but results obtained speak for themselves and promise much of 

 interest to further investigation. Thus far, only echinoids and asteroids 

 have been used as material.' 



ECHINOIDS 

 FORM AND PROPORTIONS UNDER NATURAL CONDITIONS 



Echinoid development to the pluteus larva follows, in general, the same 

 course in different groups and species with only minor differences in form 

 and proportions, skeletal pattern, etc. Cleavage, blastula, and gastrula 

 stages are figured in chapter iv (Figs. 44-46, and 49, A, B). In Figure 

 73 the gastrula {A), the prepluteus (B, C), the pluteus {D, E) of the 

 sand dollar Dendraster excentricus, and the pluteus of the sea urchin 

 Arhacia punctulata (F, G) are shown. 



■ In the following account figures are used extensively as more effective than any amount 

 of description. Most of the figures are based on ocular micrometer measurements of the di- 

 mensions of living individuals, supplemented by sketches of the same individuals. Form and 

 proportions are shown as exactly as possible, but various details of structure are often omitted 

 or diagrammatically indicated, particularly mesenchyme and skeleton. All figures of a group 

 are on the same scale. For data concerning concentrations of agents and exposure periods, 

 supplementing those given in the legends, see Appendix VIII (p. 747). 



197 



