THE VITAL PHENOMENA OF THE CELL 



227 



the axes of the nuclear spindles arrange themselves so as to be 

 parallel to the longitudinal axis of the quadrants. The same 

 thing occurs here in a somewhat modified form (Fig. 115 J5). Ou 

 account of the greater amount of protoplasm present in the upper 

 half of each quadrant, the spindle is unable to lie in the centre, as 

 in an egg in which equal cleavage occurs, but must approach 

 nearer to the animal pole of the egg. Further, it is exactly per- 

 pendicular, for, on account of the unequal weight of their halves, 

 the quadrants of the amphibian egg are firmly fixed in the egg 

 space. In consequence, the third division plane must now be 

 horizontal(Fig. 116^1), and further, it must be placed above the 



Fie. 116. Stages in the cleavage of Petromyzon. 

 after Shipley; C, D, after M. SchuHze.) 



(From Hatschek, Fig. 72 ; A, B 



equator of the sphere of the egg, being situated more or less 

 towards the animal pole. The portions thus produced are very 

 dissimilar both in size and constitution, and this is why this form 

 of cleavage has been called unequal. The four upper portions 

 are smaller, and poorer in yolk ; the four lower much larger, 

 and richer in yolk. They are called animal and vegetative cells 

 according to whether they are directed towards the animal or 

 vegetative pole. 



As development proceeds (Fig. 116 B, C,D), 

 the difference between the animal and the 

 vegetative cells grows greater and greater, 

 for the more protoplasm a cell contains, the 

 more quickly and frequently does it divide, 

 as has been already mentioned above. 



Unequal cleavage can also occur in oval 

 eggs. For instance, the egg of Fabricia 

 (Fig. 117), as has been already mentioned 

 (Fig. 109), in consequence of the collection 

 of yolk around one pole, divides into one cells. (After Haeckei.) 



