II. i CELL-DIVISION 37 



obliquely, at 45. la these eggs the yolk sinks slightly from 

 the upper to the lower side, while the cytoplasm rises in the 

 opposite direction ; in other words, a bilateral symmetry is con- 

 ferred upon the egg by the combined action of pressure and 

 gravity. The plane of this symmetry is midway between and 

 parallel to the plates. The first furrow is at right angles to the 

 plates and to the plane of symmetry. 



We are indebted to Driesch for a ^imilar series of experiments 

 on Echinoderm eggs. Driesch compressed the eggs of Echinus 

 under a cover-glass supported by a bristle. The direction of the 

 egg-axis with regard to the pressure was not known, but the 



a 



FIG. 22. Echinus: segmentation under pressure. 

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

 division (tangential) ; V , 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.) 



Echinoid egg is nearly isolecithal. When the egg membrane 

 remained intact the first two furrows were vertical, that is, in 

 the direction of the pressure, since the slide and cover-glass were 

 horizontal, and at right angles to one another. 



The spindles for the next division are again horizontal, and 

 usually tangential, sometimes, however, radial. The eight-celled 

 stage consists, therefore, of a flat plate of cells. At the next division 

 the formation of micromeres -which would ordinarily occur at this 

 moment is suppressed ; the spindles are horizontal and radial, 

 the furrows, therefore, vertical and tangential (Fig. 22 a, I, b'). 



In certain cases cell-formation is wholly or partially sup- 

 pressed. When the pressure is less (in those eggs which lie 

 nearer the bristle) the micromeres may be, but generally are not, 

 formed. The spindles are no longer horizontal. Similar results 

 are obtained when the eggs are released from strong compression. 



In another experiment the eggs were first deprived of their 

 membranes. The first and second furrows are vertical and 

 generally at right angles to one another. Sometimes, however, 



