EXOGASTRULATION AND RELATED MODIFICATION-. 



73 



readily obtained by the lithium salts and other agents, and apical 

 inhibition is evident in the characteristic reduction or entire 

 suppression of the preoral ciliary field, etc., yet the endogastrula- 

 tion process is not so easily interfered with. 



But partial and occasionally fairly typical exogastruhe occur 

 with great regularity in Asterias cultures in N/ioo to N/2OO LiCl 

 solutions in KNC, in diluted sea water, etc. (Fig. 8). 



In Orthasterias N 160 LiCl sometimes induces exogastrulation 

 or marked change of proportions of the germ layers, and not un- 

 commonly in later development the stomadeal pouch is turned 

 outward and forward from the mouth by a process apparently 

 similar to exogastrulation (Fig. 9.). 



/> C 



FIG. 9. Alterations of Orthastrrias embryos in M/i6o LiCl. d-t, showing sterna- 

 l-version; t.witli mixed endo-and exo-gastrulation; j-6.with reduced ectodcim 

 and greatly enlarged endodi-rm. particularly in the fore-gut. Note the position ot 

 the c-iliary bands in c-c and in Fit;. 8. c. 



Comparison of Susceptibility gradients of Normal and Litliittm 



Embryos. 



i. The Xormal Embryos. The susceptibility relations in the 

 i ,nl\ ontogeny of Asterias and Arbacia have been described by 

 Child ('i5&; '160); these observations have- been confirmed and 

 extended to include the sand-dollar (data by I >r. Child and tin- 

 writer) and the Pacific coast forms. 



According to the direct susceptibility method p. <\\] tin- fertil- 

 ized or dividing eggs, blastulcT?, gastrula- and larvae were placed 

 in loth. il solutions of KNC (M iooi,,M rooo), HgCl M ->ooo 

 or less), CuSOi, concentrated lu-utral red or methylene blue, and 

 to a few other salts, etc. and the course and relative times of 

 death of parts noted by the criteria of pi-ibility of recovery in 

 return to sea water and of disoruani/ation of cell structure. 

 Disorganization is often made more \i-ible either by a brief 

 urn to sea water, or by previous or simultaneous staininy in 



