216 THE CELL 



a larger amount of protoplasm than in the other half, where they 

 are larger and more closely packed together. 



In other cases a small portion of protoplasm, more or less free 

 from yolk, has separated itself from the yolk-containing portion of 

 the egg, and, as in birds and reptiles (Fig. 108 k, sch), has assumed 

 the form of a disc. 



The two poles in an egg are distinguished from one another by 

 the names animal and vegetative ; at the former most of the proto- 

 plasm collects, and at the latter most of the yolk substance ; hence 

 the former has a smaller specific gravity than the latter. In 

 consequence, eggs in which polar differentiation has occurred must 

 always endeavour to attain a certain position of equilibrium. 

 Thus, whilst in small cells, in which the substance is equally 

 divided, the centre of gravity coincides with the centre of the 

 sphere, the result being that the eggs can readily take up different 

 positions, in eggs, on the other hand, in which polar differentia- 

 tion has taken place, the centre of gravity has become eccentric, 

 having approached the vegetative pole to a greater or less degree. 

 Hence the egg so arranges itself in space that the animal pole is 

 directed upwards, and the vegetative downwards. A line joining 

 the two poles, the egg-axis, must, if the egg is allowed to move 

 freely, assume a perpendicular position. 



Frogs' eggs and Hens' eggs furnish us with useful examples of 

 this. In the Frog's egg (Fig. 115) the unequal portions can be 

 clearly distinguished externally, since the animal part is pigmented 

 and of a deep black colour, whereas the vegetative is whitish 

 yellow in appearance. If such an egg is placed in water after 

 fertilisation has occurred, in a few seconds it takes up a position 

 of equilibrium, the dark side being always turned upwards, and 

 the specifically heavier light side downwards. 



Similarly, in whatever way a Hen's egg (Fig. 108) may be 

 turned about, the germinal disc (k, sch~) will be seen to occupy the 

 highest point in the yolk sphere, for the latter rotates in its 

 albuminous sheath with every movement, keeping its vegetative 

 pole always directed downwards. 



Polar differentiation occurs both in oval and spherical eggs. 

 The egg of the worm Fabricia (Fig. 109) may serve as an example. 

 Here, at the one end more protoplasm is seen, at the other more 

 yolk substance. 



In eggs with polar differentiation it is useless to look for the 

 cleavage nucleus in the place where it is seen in eggs poor in yolk. 



