212 



THE EVOLUTION OF MAN. 



we therefore find that there are now eight cleavage-cells, 

 the descendants in the third generation of the parent-cell 

 (Fig. 40). Four larger, brighter, and firmer cells lie in one 

 plane ; the descendants in the second generation of the 

 exoderm mother-cell. Four smaller, darker, and softer cells 

 lie in a second plane, perpendicular to the former ; the 

 descendants in the second generation of the entoderm 

 mother-cell. If we connect the central points of the oppo- 

 site cleavage-cells of one plane, two and two, by straight 

 lines, these lines meet each other at right angles. But the 

 four connecting lines of the two parallel planes together 

 intersect at an angle of forty -five degrees (Fig. 40). 



Fig. 39. — The four first cleavago-cells of a Mammal (Eabbit) : e, the 

 two exodcrm-cells (larger and brighter) ; ?, the two entoderm-cells (smaller 

 and darker) ; z, zona pellucida ; h, outer albuminons envelope. 



Fig. 40. — Egg of Mammal (Rabbit), with eight cleavage-cells: e, four 

 exoderm-cells (larger and brighter) ; i, four cutoderm-cells (smaller and 

 darkei-) ; z, zona x>elhicida ; h, outer albuminous covering. 



Now, however, the eight cleavage-cells alter their 

 original ])osition, and lose their globular form. One of the 



