48 CLEAVAGE II 



really, though invisibly, bilateral. In ordinary fertilization 

 this weak primary bilaterality is superseded by the far 

 stronger bilaterality imposed by the sperm, a bilaterality 

 which persists as that of the embryo. In artificial par- 

 thenogenesis the primary bilaterality remains, becoming 

 manifest as the result of the stimulation. 



It must be pointed out that the number of cases on which 

 Brachet relies are really too few to support any safe conclu- 

 sion, and further that the coincidence between the plane of 

 symmetry of the fertilized egg and the median plane of the 

 embryo is not absolute but only approximate, as the accom- 

 panying table of the frequencies of the angle between the two 

 planes will readily show (Table VI). The eggs used in this 

 experiment were placed on the slides with their axes vertical 

 and the white pole below, to avoid any disturbing influence 

 of gravity, and spaced to avoid the influence of pressure. 



TABLE VI. FREQUENCY. 



Angle. Positive. Negative. Total frequency. 



0-15 68 59 127 



15-30 36 32 68 



30-45 10 13 23 

 45-60 459 

 60-75 189 



75-90 3 1 4 



90-105 1 1 



105-120 



120-135 1 1 



135-150 



150-165 2 2 



165-180 1 1 



It is clear that while there is a very strong tendency for 

 the two planes to coincide, it may happen that they diverge 

 a good deal, even to the extent of 180. 



Some caution should therefore be exercised in drawing con- 

 clusions as to the coincidence of the planes of fertilization, 

 egg-symmetry, and embryonic symmetry from a small number 

 of observations. 



2. The second problem presented by cell-division is con- 

 cerned with the causes which determine the particular pattern 

 of cleavage in each case. 



