EXOGASTRULATION AND RELATED MODIFICATION-. 



sea water do -how -ome of .the lithium modifications much ob- 

 -t tired by recovery. In general it maybe said that maximal 

 effect- may be- -ecured by sea water solutions of about X no to 

 X joo I.i< inline to the species of the embryo used) acting 



I'm- .1 sensitive determining period from cleavage continuously 

 through late blastula stages. 



Echinarachnius parma. This sand-dollar provides perhaps 

 ilu nio-t re-ponsive material yet found for the study of lithium 

 t-l h-fts, which are here, if anything more diverse and extreme 

 than in sea-urchins and far more so than in the star hshes used. 



The action of lithium solutions producing the characteristic 

 lithium effects is such as to retard or inhibit development, the 

 ree of inhibition (and loss of power of recovery) varying 

 directly with concentration. I.iCI, 2 H- : < ). X 60 or more, in 

 \\..ier stops cleavage early; and even at X 80 only a few individ- 

 uals reach the blastula stage. These survivors are either partial 

 blastula- or very small, immobile, thick-walled stereo-blastul.i . 

 \\liose remarkably small segmentation cavity is packed so full 

 of precociously formed opaque mesenchyme that much of the 

 latter is forcibly extruded from the basal pole; these types, 

 though not "sickly," die shortly without gastrulation. In 



X i leath occurs most commonly in late cleavage, but one 



timK inactive at the bottom of the container, a few stereo- 

 blastuLe. which die as such, or, rarely, after assuming the con- 

 striction form described below. 



In X i JS I.iCl a considerable minority of the egt- In. dine 

 opaque mesenchyme-filled blastuhe, the larger of which, e-pe- 

 cially in more <lilule (X 150) Lid, often show a \\ide /one ol 

 thickening of the basal pole and after some delay bev;in t !u- elonga- 

 tion preluding gastrulation; but the entoderm then eNa^inate- 

 after the fashion of that in lithium < 'nila-. There i- much 



variation in the diameter of the rin- u~ual!\ ba-al ' where the 

 ectodermal (gastrula wall) and entodi-i in.il iai - i henteron \ |)or- 

 tions of the embr\o join, but tin- riir^ i- aKvay- enlarged "tlen 

 so much so that it mo\-e- toward, to. or e\ en pa-t the equator of 

 the blastula (Fig. I. ;/-</); in the extreme cases the e\o-astrula- 

 tion process appears to be niereU a con-trictioii bei\\een the 

 animal and \egetal |ole>, forming two vesicles, one 



