KXOGASTRULATIOX AND RELATED MODIFICATIONS. 65 



pertrophy, until in a few individuals by its progressive enlarge- 

 ment the whole gastrula is entodermal (i.e. a "holentoblastula," 

 better called for its physiological equivalent a holentogastrula). 

 There i> never any real difficulty in distinguishing ectoderm 

 and entoderm in these forms. In every species there are charac- 

 teristic differences in pigmentation, in thickness of walls, in 

 structure of the cells, in cilia, in size of cells, in frequency of 



a f> c d c f g ' 



FIG. 2. Series of sand-dollar embryos (constriction forms) developed 27 hours 

 in M/i6o LiCI. Ectoderm (above) separated at dashed line from cnd"'k-im. 

 Mcsenchyme packing the blastocoels omitted. The ectodermal (thin-wall, .h 

 component, larger in a-c, is reduced in size in d-f, and nearly obliterated in >;, 

 \\liu li is practically a holentogastrula. 



nuclei, etc. From a study of the lineage of cells from the pie- 

 men ted and nonpigmented zones of the early egg it may be stated 

 quite positively that entoderm grows at the expense of ectoderm, 

 some of whirh becomes converted into entoderm (and possibly 

 aUo into mesenchyme). 



I ill in the solution many of these constriction forms show 

 early death and gradual disintegration, sometimes of the apir.il 

 eetoderm, or of both extremes, but usually of the entodermal 

 component from its free end towards the an. u lied p. in. 1< - 

 turned to sea water the more resistant ectoderm. il portion may 

 recover and linger on awhile as a partial form, a holectogastrula 

 as it were. This type may arise al>o in .mother way: if the con- 

 stricted gastrulae are removed early from the lithium and allowed 

 to recover in sea water ectoderm and entoderm tend to separate 

 from each other, especially when the two eoinpoiieiii^ are of 

 nearly equal size (Fig. 3, v). It would ,-eeni that equality and 

 duality are inconsistent with development of mie individuality! 



N/l6o is close to the optimal concentration for producing the 

 most striking lithium effects in thi> >perie-. Retardation of 



