EMBRYONIC INDUCTORS AND ORGANIZERS 437 



creasing in concentration respectively basipetally and acrope tally, but as- 

 sumes that after isolation of parts ectodermal substance accumulates 

 apically, entodermal substance basally, and that transplanted micro- 

 meres are not organizers, as Horstadius maintains, by production of "ento- 

 dermal" substance, but merely attract it. 



These hypotheses are attempts to interpret sea-urchin development and 

 its experimental modifications in terms of formative substances. They 

 present interesting similarities to the attempts made by Morgan to ac- 

 count for reconstitution in Tubularia and other forms by assuming two 

 opposed material gradients and the changes in them required by the ex- 

 perimental data (Morgan, 1905, and various earher papers). It will be 

 recalled from earlier chapters that differential dye reduction in three sea- 

 urchin and one starfish species and differential susceptibility, as indicated 

 by differential death and differential modification of development, sug- 

 gest a somewhat different view. These data indicate presence in early 

 stages of only one gradient, corresponding in direction to the animal gra- 

 dient of Runnstrom; but preceding gastrulation a second gradient with 

 the same characteristics as the primary, as regards susceptibihty and dye 

 reduction, appears in the basal or vegetal region. The high end of this 

 gradient is distinctly higher than the high apical end of the primary gra- 

 dient ; and, as immigration of primary mesenchyme occurs, its cells become 

 the most rapidly reducing cells of the blastula. A ventrodorsal gradient 

 also becomes evident before gastrulation. The secondary gradient does 

 not overlap the primary but simply obliterates and reverses gradient di- 

 rection for a greater or less distance acropetally. Again it must be noted 

 that these susceptibility and reduction gradients do not constitute evi- 

 dence either for or against the overlapping substance gradients, though 

 they perhaps indicate that the difference of apical and basal metabolism 

 is not as great as Runnstrom and Lindahl believe ; they certainly indicate 

 that change in gradient pattern is a feature of normal development. That 

 different substances are present at different levels of the polar axis is indi- 

 cated by the pigment band of certain species (Boveri, 1901) and by dark- 

 field observations (Runnstrom, 1928a), but visible granules and sub- 

 stances appear, in general, to be primarily results of more fundamental 

 physiological differences along the axis. Unquestionably, a change in phys- 

 iological condition in the basal region takes place preceding gastrulation, 

 and immigration of mesenchyme and invagination of entoderm appear 

 to be associated with this change. Both susceptibility and dye reduction 



