42 . GEORGINA B. SPOONER 



yolk layer, because of its relative density, may have much the 

 same influence upon the karyokinetic figure as does the cell 

 wall separating two blastomeres of the two-cell stage. Spindles, 

 therefore, which start to form at an angle to the stratification, 

 would tend to slip around into a position in the long axis of the 

 protoplasmic layer and consequently parallel to the stratification. 

 The cases of parallel cleavage then would be those in which the 

 male pronucleus approaches the female pronucleus in a direction 

 parallel to the layers ; and the centrosomes pass around the nucleus 

 in such a way that the spindle forms directly across the proto- 

 plasmic band perpendicular to the layers and is so symmetrically 

 placed that it cannot slip around into the longer axis. Such an 

 explanation accounts for the very small percentage of parallel 

 cleavages. If the spindle starts to form obliquely, the proto- 

 plasmic movements are unequal about the aster and these centers 

 tend to move into more radially symmetrical positions in the long 

 axis. This accounts for the conspicuous absence of oblique 

 cleavages after centrifuging. 



Effect of centrifuging on the spindle 



In the experiments relative to the mortality under various 

 conditions, described above (table 1), two sets were centrifuged 

 respectively forty-five and fifty minutes after fertilization at a 

 time when the first cleavage spindle had formed in all the eggs. 

 The percentage of mortality in these two sets was even less than 

 in the control. Again a set of eggs was centrifuged forty minutes 

 after fertilization and a single field was observed under the micro- 

 scope until the 84- or 128-cell stage. Of ninety-five eggs in which 

 cleavage could be seen all divided perpendicularly to the layers 

 and eighty-eight developed to the late cleavage stages. The 

 others divided irregularly and died; twelve eggs, which had 

 divided perpendicularly to the stratification within ten minutes, 

 were isolated. One died soon after isolation. The other eleven 

 developed to the stage of swimming gastrulas. The development 

 of the egg is apparently not interfered with by centrifuging in 

 these stages. 



