CRITIQUE OF THE ROUX-WEISMANN THEORY 



411 



from the normal (Fig. 186); yet such eggs when released from press- 

 ure continue to segment, without rearrangement of the nuclei, and 

 give rise to perfectly normal larvae. I have repeated these experi- 

 ments not only with sea-urchin eggs, but also with those of an annelid 

 (Nereis), which yield a very convincing result, since in this case the 

 histological differentiation of the cells appears very early. In the 

 normal development of this animal the archenteron arises from four 

 large cells or macromeres (entomeres), which remain after the suc- 

 cessive formation of three quartets of micromeres (ectomeres ) and the 

 parent-cell of the mesoblast. After the primary differentiation of 

 the germ-layers the four entomeres do not divide again until a very 

 late period (free-swimming trochophore), and their substance always 

 retains a characteristic appearance, differing from that of the other 



Fig. 186. Modification of cleavage in sea-urchin eggs by pressure. 



A. Normal eight-cell stage of Toxopneustes. B. Eight-cell stage of Echinus segmenting under 

 pressure. Both forms produce normal Plutei. 



blastomeres in its pale non-granular character and in the presence of 

 large oil-drops. If unsegmented eggs be subjected to pressure, as in 

 Driesch's echinoderm experiments, they segment in a flat plate, all 

 of the cleavages being vertical. In this way are formed eight-celled 

 plates in which all of the cells contain oil-drops (Fig. 187, D\ If 

 they are now released from the pressure, each of the cells divides in 

 a plane approximately horizontal, a smaller granular micromere being 

 formed above, leaving below a larger clear macromere in which the 

 oil-drops remain. The sixteen-cell stage, therefore, consists of eight 

 deutoplasm-laden macromeres and eight protoplasmic micromeres 

 (instead of four macromeres and twelve micromeres, as in the usual 

 development). These embryos developed into free-swimming trocho- 

 phores containing eight instead of four macromeres, which have the 

 typical clear protoplasm containing oil-drops. In this case there can 



