II. i CELL-DIVISION 39 



plates and the division horizontal. Ziegler points out that, in 

 the former cases, the cells have greater dimensions in the hori- 

 zontal plane than in the latter. This, however, may be the 

 effect, not the cause, of the direction of the spindle-axis. 



Two other pressure experiments may be mentioned here. In 

 Nereis Wilson produced a flat plate of eight equal cells by 

 applying- pressure in the direction of the axis. The formation 

 of the first quartette of micromeres was thus suppressed. On 

 relieving the pressure eight micromeres were formed. For the 

 Ctenophora (Beroe) Ziegler has shown that the normal inequality 

 of the third and fourth divisions is not altered by pressure. 1 



The foregoing experiments all agree in demonstrating the 

 perfectly definite effect produced by pressure upon the segmenting 

 egg. The nuclear spindles place themselves at right angles to 

 the direction of pressure, the divisions fall at right angles to the 

 compressing plates. This holds good for the first three or four 

 divisions, at least, and sometimes for later phases still. In all 

 these cases, therefore, the nuclear spindle elongates in a direction 

 of least resistance, and, in the normal uncompressed egg, we may 

 argue, with Hertwig, the least resistance is offered by the greatest 

 protoplasmic mass. 



Even in the compressed eggs, however, the greatest extension 

 of the protoplasm, or the least extent of the yolk, is a factor 

 which must in some cases come into play. When the egg of the 

 Frog is compressed between vertical plates, the nuclear spindle 

 does not elongate in any direction at right angles to the pres- 

 sure, but in one only, a horizontal ; and this is the direction of 

 the greatest protoplasmic mass, since the egg-axis is vertical. 



Speaking generally, therefore, experiment has upheld Hert- 

 wig's contention that the direction of nuclear division, and 

 therefore of cell-division, is determined by the relation between 

 the nucleus with its centrosomes and the cytoplasm with its yolk. 



There are one or two experiments which do not support 

 Hertwig's view. Boveri stretched the eggs of the sea-urchin 

 Strongylocentrotm in the direction of the axis. The fertilization 

 spindle lay in the usual equatorial position, occupied, that is, the 

 minor axis of the ellipsoid. 



1 I have recently had occasion to notice that when the egg of Antedon 

 is compressed in the direction of the axis the third division is meridional 

 instead of latitudinal. 



