Experimental Studies on Germinal Localization. 253 



Boverl ('oi.i) has In fact shown experimentally that the ability 

 to gastrulate depends on the presence of a certain amount of the 

 pigment-band that approximately coincides with the entoblast- 

 zone; and the variation in this regard is explicable under the as- 

 sumption either of a varying position of the third cleavage-plane 

 with respect to the entoblast-zone or of a variation in the de- 

 gree of concentration of the entoblast-stuff. While Boveri adopts 

 provisionally the former of these alternatives, he also suggests 

 that the formation of entoblast and mesenchyme is not absolutely 

 predetermined in the plasma, but occurs at the "most vegetative" 

 point, which is the lower pole. Driesch ('02.2) adds the sug- 

 gestion that the frequent failure of the animal larvae to gastrulate 

 may be due, not to absolute lack of "vegetativlty" ("Um einen 

 nicht sehr schonen aber deutllchen Ausdruck zu gebrauchen") , 

 but to its Insufficient degree; and he has recently shown ('03) by 

 an experiment of admirable ingenuity that artificial displacement 

 of the third cleavage-furrow towards the vegetative pole causes 

 a large Increase in the proportion of gastrulas produced by the 

 isolated upper cells. This interpretation becomes perfectly intel- 

 ligible If stated frankly in terms of the formative stuff hypothesis; 

 and it harmonizes with my conclusion regarding the Dentalium 

 egg that the influence of the specific stuffs is within certain limits 

 qualitative rather than quantitative, which was based on the fact 

 that If the upper part of the egg be cut away, leaving the whole 

 of the lower pole area the polar lobe typically is reduced to the 

 correct proportional volume, and the resulting larva has a post- 

 trochal region of the proper size. This conclusion Is in agree- 

 ment with that of both Boverl and Driesch, that the plasma struc- 

 ture plays "nur eine determlnirende, kelne fixierende Rolle" 

 (Driesch, '02.2, p. 522). 



In the ctenophore It appears from Fischel's observations ('98) 

 that the first qualitative division is the fourth, which first sep- 

 arates ectoblastic micromere material from the entoblast con- 

 taining basal cells. In the whole series up to this point we have 

 been considering a segregation that in its initial form is vertical 

 and symmetrical about the axis, though in the moUusk and an- 

 nelid it becomes asymmetrical in the course of the first cleavage 



