EMBRYONIC INDUCTORS AND ORGANIZERS 445 



least equally hypothetical. Micromeres are supposed to increase the vege- 

 tal gradient. How do they determine an animal gradient opposite in di- 

 rection to the original in the basal region of the half-blastula? The gra- 

 dient changes required can, of course, be assumed to take place; but such 

 assumptions are certainly no less hypothetical than those concerning 

 physiological isolation, for physiological isolation is demonstrated in many 

 cases. 



As already noted, development of mesenchyme and entoderm from the 

 basal regions of apical halves is brought about by certain exposures to 

 Kthium. The most basal regions of lithium-treated apical halves, marked 

 by local staining at the time of isolation, when implanted in the basal 

 regions of other apical halves, prevent the extreme apical partial develop- 

 ment characteristic of isolated apical halves and induce entoderm forma- 

 tion, and development of essentially normal plutei may result. These im- 

 planted cells are regarded as a secondary organizer by Horstadius (19366). 

 Cells from the apical regions of lithium-treated blastulae, similarly im- 

 planted in apical halves, perhaps have slight effect in inhibiting extension 

 of the apical tuft; but, since they do not induce entoderm, they afford 

 evidence that the effect of the "secondary organizer" is not due to pres- 

 ence of hthium in it. Horstadius regards the action of hthium as similar 

 to that of implanted micromeres, but the evidence from chapter vi indi- 

 cates that lithium is generally inhibitory ; and the production of entoderm 

 in the lithium-treated apical halves and in those with implanted basal 

 cells from hthium-treated apical halves is apparently quite similar to the 

 reconstitution of more basal parts in short pieces of Tuhulana and Cory- 

 morpha when scale of organization is decreased by inhibiting conditions 

 (pp. 344-48). Activation of mesenchyme and entoderm in the lithium- 

 treated half occurs in recovery after return to water, not as a direct effect 

 of lithium. Heteroplastic transplantation of micromeres has shown that 

 the inducing action and the reactions of the host ectoderm to the skeleton 

 are not species-specific and that the skeleton in the resulting chimeras 

 may possess characteristics of the donor species.' 



In normal asteroid development there are no micromeres, nor is any 

 mesenchyme formed preceding gastrulation ; but invagination of ento- 

 derm occurs in essentially the same way as in the sea urchin, though the 

 most intense activation of the entoderm apparently takes place after in- 

 vagination (p. 137). Even in the sea urchin presence of micromeres or of 

 primary mesenchyme, resulting from reconstitution of other parts of the 



3 Von Ubisch, 1931, 1932(7,6, 1934, igi^^h; Horstadius, iq^ed; F. J. Schmidt, 1936. 



