HABITS, ETC., OP CERBBBATULUS LACTEUS. 141 



Segmentation. 



Segmentation is total and equal. The first furrow lies 

 along an axis joining the polar bodies and the point of 

 original attachment. This fact furnishes further evidence 

 that the axis of segmentation was determined early in deve- 

 lopment. About an hour and a quarter after fertilisation 

 the egg begins to elongate at right angles to the axis just 

 mentionedj thereby drawing the polar bodies away from the 

 membranes. It continues to elongate until the long dia- 

 meter becomes one and a half times the shorter one. 



If viewed at this period under a low power with strong 

 illumination, it can be seen that the nucleus has divided, and 

 the halves have migrated to either end of the ellipse. 



A shallow groove appears beneath the polar bodies and 

 the egg elongates still more, the diameters bearing the rela- 

 tion to each other of five to eight. Two minutes later a 

 corresponding groove appears on the opposite side, and they 

 both deepen rapidly, the first more rapidly than the second. 

 There is also a continued elongation, the diameters standing 

 in the relation of five to nine. At the end of this period, 

 which has lasted ten minutes, we find the egg divided into 

 two almost perfectly spherical blastomeres, joined by a very 

 narrow isthmus, whose distance from the polar bodies is 

 twice that from the opposite pole (fig. 49). 



Behaviour of the Polar Bodies. — Usually the flatten- 

 ing of the egg preliminary to segmentation moves the polar 

 bodies, but sometimes the connecting band of protoplasm 

 elongates and leaves them in their original position. When 

 the furrow appears they are always pulled down into it. 

 Under a low power it is seen that they follow the changes in 

 position which the surface undergoes as if they were attached 

 to it. 



Under higher magnification one can detect that the bodies 

 themselves are somewhat changed in contour by the process 

 (fig. 42). Instead of its characteristic spindle shape, with 

 the long diameter parallel to the surface of the egg, the 



