THE PEOGESS OF CLEAVAGE. 



59 



In the case of the Trog's egg, consequently, we find it' in the black' 

 pigmented hemisphere, which is turned upward. 



When in this case the nucleus prepares to divide, its axis can no 

 longer assume the position of any and every radius of the egg. In 

 consequence of the want of uniformity in the distribution of the 

 protoplasm, the nucleus comes under the influence of the more 

 protoplasmic pigmented part, which rests on the more deutoplasmic 

 portion like an inverted cup, and, on account of its less specific 

 gravity, floats at the surface, and is spread out horizontally. But 

 in a horizontal protoplasmic disc the nuclear spindle comes to occupy 

 a horizontal position (fig. 31 A sp). Consequently the plane of 

 division must be formed in a vertical direction. A small furrow now 



2ir 

 sp 



Fig. 31. — ^Dia^am of the division of the Frog's egg. 



A, Stage of the first division. B, Stage of the third division. The four segments of the second 

 stage of division are beginning to he divided by an equatorial furrow into eight segments. 

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

 in protoplasm ; d, the part which is richer in deutoplafim ; sp, nuclear spindle. 



begins to show itself — at the animal pole first, because the latter is 

 more under the influence of the nuclear spindle, which lies nearer 

 to it, and because it contains more protoplasm, from which proceed. 

 the phenomena of motion during division. The furrow gradually 

 deepens downward, and cuts through to the vegetative pole. 



By the first act of division we get two hemispheres (fig. 32 ^), each 

 of which is composed of a quadrant richer in protoplasm and directed 

 upward, and another poorer in protoplasm and directed' downward. 

 By this means both the position of the nucleus and the direction of ' 

 its axis are again determined, when it prepares for the second 

 division. According to the rule previously laid down, the nucleus is 

 to be sought in the quadrant which contains the more protoplasm ; 

 the axis of the spindle must take a position parallel to the long 

 axis of the quadrant, and must therefore come to lie horizontally 



