JOHN \V. MACARTHUR. 



marked capacity of inhibiting locally, retarding apical regions 

 without appreciably hindering basal regions. 



(b) Exogastrnlation. In practice quite typical exogastruhe 

 are obtained in the sand dollar or in sea-urchins by all the lithium 

 halides, sodium butyrate, KC1, etc. (Herbst), by CaCl 2 , dilution 

 or staling of sea water, etc. (this paper), by elevation of tempera- 

 ture to 30 C. during gastrulation (Driesch); and initial stages of 

 exogastrulation by main' other agents, as NaCl, CuSO 4 , neutral 

 red, etc. As regards exogastrulation specificity appears to reside 

 chiefly in the biological material rather than in the external agent, 

 for perhaps only in the pluteus-forming echinoderms is the lithium 

 effect "typical"; in the starfishes, Crepidula (Conklin), and the 

 sponge, Oscarella (Maas, '98) the effects are sufficiently marked 

 and similar as to be recognizable; and in the annelids, ascaris, 

 amphioxus, frogs (?) only negative results appear. Possibility of 

 inducing exogastrulation depends, doubtless, on the specific 

 constitution and organization of the egg, especially on the polar 

 arrangement of the levels destined to form ectoderm, entoderm 

 and mesenchyme. The physiological equivalents of exogastrulae 

 may well appear quite different morphologically in different 

 organisms. 



4. The Mode of Action of the Lithium. 



The problems of lithium action resolve into the causes of the 

 two essential modifications: first, the disproportionate develop- 

 ment of entoderm and mesenchyme; and second, exogastrula- 

 tion. These are without doubt closely interrelated processes 

 proceeding from a common fundamental cause. 



Herbst admitted that he found himself in complete darkness 

 on these matters but gave reasons for his opinion that the site 

 of action of the salt must be in the egg itself rather than in its 

 external membranes, and that Li' must produce a specific stimu- 

 lating effect on vegetal pole and entodermal cells, whose capacity 

 for taking up and retaining this ion he believed to be greatr-t 

 ('95*7, p. 185). Seeking to put the matter on a physico-chemical 

 basis one of his students (Spek. '18) gave some evidence that it 

 Li' did penetrate into certain (e.g., entodermal) cells ii svould 

 make for a greater imbibition of water by their colloids. Driesch 

 ('95) believed that "the growth process of tin- blast ula wall is 



