IV. i INITIAL STRUCTURE OF THE GERM 



193 



mesenchyme, and with an undivided arelienteron (Fig. 102); the 

 macromeres, on the other hand, become gastrulae provided with 

 mesenchyme and spicules (Fig. 103). 



Lastly, even ^ blastomeres will occasionally gastrulate, pro- 

 bably only if derived from a macromere. This gastrulation was 



a 



FIG. 102. Echinus. Larvae reared from mesomeres (animal cells) of 

 the 16-celled stage, a and b, mesenchyme gastrulae, a has spicules ; 

 c, gastrula without mesenchyme ; d, a long-ciliated blastula. (After 

 Driesch, 1900.) 



not, however, observed directly, but the germinal value of the 

 smallest gastrulae found was calculated by a method which will 

 be described below. -$ cells will not reach the gastrula stage. 



FIG. 103. Two gastrulae of Echin its reared from ^ macromeres; both 

 have mesenchyme cells, one has triradiate spicules. (After Driesch, 1900.) 



The differences between the developmental capacities of animal 

 and vegetative cells may be studied in another way. The four 

 micromeres were removed by Drieseh in the 16-celled stage ; 

 the remaining twelve meso- and macromeres produced a normal 

 Pluteus (an experiment also performed by Zoja on Strongylocen- 

 trotiis). The eight (or four in the previous stage) animal cells 

 alone, the eight vegetative cells alone also formed, in some cases, 

 a perfect larva. Segmentation was in all these cases partial. 



JESK1NSON 



