IV. i INITIAL STRUCTURE OF THE GERM 185 



' karyokinetic plane') and elongates at right angles to the sperm- 

 path, which thus lies in the plane of the first, a meridional, furrow 

 (a similar observation had been previously made by Wilson and 

 Mathews on Toxopneustets). 



The second furrow is likewise meridional and at right angles to 

 the first, the third equatorial or a little nearer to one or other pole; 

 the animal cells get only a very little of the pigment. In the 

 next phase the animal cells divide meridionally and equally to 

 form the eight mesomeres, while the vegetative cells divide 

 latitudinally and veiy unequally into four large pigmented 

 macromeres next the equator, and four small quite unpigmented 

 micromeres grouped round the vegetative pole. 



In the blastula stage all the cells are of the same size, vacuo- 

 lated and ciliated, and the polarity of the egg is only deter- 

 minable by the position of the pigment zone. In the next stage 

 the clear vegetative cells derived (presumably) from the micro- 



a 



FIG. 93. Echinus : segmentation under pressure. 

 , preparation for third division (radial) ; 6, preparation for fourth 

 division (tangential) ; &', after fourth division ; c, another form of the 

 8-cell stage (third division parallel to first) ; d, the same after removal 

 of the pressure. (After Driesch, 1893.) 



meres wander in to form the primary mesenchyme from which 

 the first triradiate spicules are developed and this is followed by 

 the invagination of the pigmented cells to form the archenteron. 

 Secondary pigmented mesenchyme cells are budded off from the 

 inner end of the latter. The character and sequence of the 

 divisions in segmentation are the same in other Echinid eggs, but 

 the polarity is not marked by pigment, nor indeed recognizable 

 till the micromeres have been formed, and again when the 

 mesenchyme cells wander into the blastocoel. 



By means of pressure under a coverglass Driesch succeeded 

 (Echinus) in altering the direction of the spindles and consequently 

 of the cell-divisions (Fig. 93). The first and second furrows were at 



