226 



THE CELL 



taining portion, and, on account of its smaller specific gravity, 

 floats upwards, and is spread out horizontally (Fig. 115^4). The 



A B 



P ^m^ ^*** P 



-pv 



Fig. 115. Diagi*aui of the division of the Frog's egg (0. Hertwig, Emhryology, Fig. 31): 

 A first division stage ; B third division stage ; the four portions of the second stage 

 of division are beginning to be divided by an equatorial furrow into eight portions; P 

 pigmented surface of the egg at the animal pole; pr that part of the egg which is richer 

 in protoplasm ; d that part of the egg which is richer in deutoplasm ; sp nuclear spindle. 



nuclear spindle, however, lies horizontally, in a horizontal disc of 

 protoplasm ; hence the division plane must develop vertically. 

 At first a small furrow appears at the animal pole, since this 

 latter is especially influenced by the nuclear spindle which has 

 approached it, and further because it contains more protoplasm, 

 in which the movements occurring during division commence. 

 The furrow slowly deepens, cutting downwards towards the 

 vegetative pole. 



The two hemispheres produced by this first division consist of 

 an upper portion, rich in protoplasm, and of a lower portion, poor 

 in protoplasm. By this means, in the first place the position of 

 the nucleus, and in the second place its axis, are absolutely de- 

 termined before it commences to divide a second time. The 

 nucleus is to be looked for, according to the above-mentioned law, 

 in that quadrant which is richest in protoplasm. The axis of the 

 spindle must here lie parallel to the longitudinal axis of the 

 quadrant, that is to say, it must lie horizontally. Hence the 

 second division plane, like the first, is perpendicular, cutting 

 the latter at right angles. 



At the end of the second cleavage the amphibian egg consists 

 of four quadrants which are separated from one another by verti- 

 cal division planes, and which possess two unequal poles, the 

 upper one being lighter and richer in protoplasm, and the lower 

 one heavier and richer in yolk substance. In an egg where equal 

 cleavage occurs, we saw that at the stage of the third division 



