DIFFERENTIAL DEVELOPMENTAL MODIFICATION. II 211 



of development of entoderm is highly variable: in some, all three regions 

 are present; in others, two; while in still others it is a spherical vesicle. 

 Very often it separates from the region of the blastopore, of which no 

 trace remains, and usually it comes to lie near the secondarily modified 

 apical region. The secondary apical modification consists in outgrowth, 

 often with development of ciliated band epithelium, in development of a 

 mouth, or occasionally only in apparent attachment of entoderm to the 

 apical ectoderm. If a ciliated band develops, it is basal. The blastocoel 

 contains cells in varying number (not figured), the mesenchyme, and in 

 many individuals cells dissociated from prospective entoderm, but no 

 skeleton develops. Figures 81 and 82 represent the most advanced stages 

 of these forms, but they often live longer without further development 

 than fully developed plutei from the same lots of eggs."* 



ASTEROIDS 



Differential modifications of asteroid development show the same rela- 

 tions as those of echinoids to the gradient pattern indicated by other 

 methods. In differential inhibition apical and ventral regions of ectoderm 

 and apical region of entoderm are most inhibited, and in the secondary 

 modifications these regions become relatively large. 



EARLY LARVAL FORM UNDER NATURAL CONDITIONS 



The gastrula of the starfish Patiria miniata was outlined in Figure 49, 

 C-E, of chapter iv; a later stage is shown in Figure 83; development of 

 Asterias is similar. As a basis for consideration of the differential modifi- 

 cations it is important to note the following points of difference between 

 asteroid and echinoid development: first, in the asteroid the mesenchyme 

 does not immigrate before gastrulation but is first formed from the apical 

 entoderm after invagination is completed and increase in rate of dye re- 

 duction in the enlarged apical entoderm has occurred (Child, 19366); 

 second, the invaginated asteroid archenteron extends only about halfway 

 from the basal to the apical pole; third, instead of becoming a flattened 

 oral lobe, as in echinoids, the apical region of the starfish develops into a 



4 These types of secondary modification have been obtained with alcohol, acid and hypo- 

 tonic sea water, CuSO^, HgCL, ultra-violet radiation, and visible hght with sensitization by 

 eosin. Differential tolerance or conditioning to KCN is shght, but differential recovery gives 

 the whole series of forms after certain ranges of concentration. Similar secondary ectodermal 

 modifications occur with LiCl, but very commonly in association with some degree of exogas- 

 trulation. 



