EMBRYONIC INDUCTORS AND ORGANIZERS 



439 



ments the possibility exists that calcium-free sea water, isolation of micro- 

 meres or other blastomeres or groups, staining of particular cells or groups, 

 and the manipulation necessary for transplantation may alter physiologi- 

 cal condition of the cells concerned, either in the direction of activation 

 or of depression; but whether such change occurs is not known. That it 



Fig. 146, .4 -i/.— Development of isolated apical halves of ParacentroUis. A, isolated apical 

 half; B, C, earlier and later stages of e.xtreme apical partial development; D, E, earlier and later 

 stages of less extreme apical development; F, G, H, essentially normal development follow- 

 ing implantation of four micromeres in basal region of apical half of early blastula (after 

 Horstadius, 1935). 



may be a factor in the variation of results of a particular experiment 

 seems not impossible. 



Apical halves, isolated in cleavage stages (Fig. 14^, A), usually develop 

 into thick-walled, blastula-Hke forms with extension of the apical tuft of 

 long stiff cilia over a large part of the surface (Fig. 146, B). These are 

 extreme apical partial forms, comparable to the apical partial forms from 

 short pieces of Tuhularia and Corymorpha (Fig. 113, A-I [p. 334])- Later 



