JOHN W. MACARTHUR. 



plan of organization of the Echinid egg as such that the prospec- 

 tive significance of different levels of the egg along its main struc- 

 tural axis is as follows (see Przibram, '08): The animal pole 

 half (diagram, Fig. n. Ect.) furnishes the ectoderm and its 

 differentiations; the subequatorial zone (End.) the gut and its 

 derivatives; and the vegetal pole (Mes.) the primary mesenchyme 

 and the larval skeleton. In Fig. n (6) and (c) these parts are 

 homologized in the normal and the lithium-modified gastrula? 



a 



y 



FIG. 12. Disintegration of typical lithium-modified sand-dollar embryos and 

 larv;f. Mesenchyme omitted, a-c in neutral ivd; </ /, in HgCli M/sooooo; j-q, 

 in HgCl: plus Neutral Red; r-y, in KNC, M/IOOO; /, q, y, fused doubles. In the 

 less modifier! forms (a, b, d, j, r) susceptibility is about equal apically and basally. 

 In the more modified forms with large development of endoderm a distinct basi- 

 apical disintegration gradient is always found. In general the larger the relative 

 size of the endoderm and of the fore-gut the more distinct is this reversed gradient. 



