190 



INTERNAL FACTORS 



IV. i 



also indebted to Driesch for a long series of experiments on the 

 behaviour of isolated blastomeres and egg-fragments. In the 

 forms which we have hitherto considered the Vertebrates and 

 Coelentera an isolated cell segments as a whole and gives rise 

 to a total larva. In these Echinoderms the isolated blastomere 

 also gives rise to a total larva, but its segmentation is partial ; 

 only after segmentation has been completed does the open blastula 

 close up and resume the polarity of a whole. 



Various methods have been employed for separating the 

 blastomeres. Loeb showed that in dilute sea-water (50 %) the egg 



FIG. 98. Development of the isolated -J blastomere of Echinus microtu- 

 berculatus. Two micromeres, two macromeres, four mesomeres : , | ; &, ^ g ; 

 c, Hemiblastula : on the right is the remaining ^ blastomere, dead. (After 

 Driesch, 1892.) 



swells, bursts its membrane and protrudes an ex-ovate which 

 may be large and develop independently (Fig. 97). Driesch has 

 used heat, pressure, violent shaking with fragments of cover- 

 glasses, and the calcium-free sea- water introduced by Herbst. 



The isolated \ blastomere in Echinus becomes rounded and seg- 

 ments as a half, as though the other blastomere were still present. 

 It forms four mesomeres, two macromeres, and two micromeres, 



