168 



INTERNAL FACTORS 



IV. i 



at right angles to the axis, invariably separates anterior from 

 posterior. 



An observation of Morgan's has some bearing on this question ; 

 even when the first furrow divides the egg unequally a normal 

 embryo results. 



Even when the first two blastomeres are equal the cells pro- 

 duced by their subsequent division may migrate across the plane 

 of separation (Kopsch). In Necturus there appears to be no relation 

 between the first furrow and the sagittal plane (Eycleshymer). 



The hypothesis of qualitative nuclear division has been tested 

 by O. Hertwig in another way. 



FIG. 82. The first four phases of segmentation of the Frog's egg. 

 Normally (the three upper figures) and under pressure between hori- 

 zontal plates (the three lower figures) I, //, III, IV, the first, second, 

 third, and fourth divisions. 1-16, the cells produced by successive divi- 

 sions, numbered as follows : 



The egg divides into 



The animal cells in the normal egg and the corresponding cells in the 

 compressed egg are stippled. (After 0. Hertwig's account.) 



The eggs are affixed to glass plates by their jelly, fertilized, 

 and allowed to rest until the axis has become vertical. Pressure 

 is then applied by a second plate, in the direction of the axis, 

 the plates being horizontal, or at right angles to the axis, the 

 plates being vertical or oblique. In such compressed eggs the 

 direction of elongation of the nuclear spindles is distorted-, and 



