CH. IV] CLEAVAGE OF THE EGG 37 



The nucleus of each blastomere during the resting-period 

 undergoes a series of changes, the so-called reconstructive pro- 

 cess taking place. The chromatin-granules or chromosomes 

 are again surrounded by a nuclear membrane, and the granules 

 fuse into a thread or network. At the next division of the 

 egg the nuclear chromatin is again set free in the protoplasm 

 by the absorption of the nuclear membrane. A spindle is 

 formed and the chromatin in each cell is again exactly halved. 



The second cleavage-furrow appears about three-quarters of 

 an hour after the appearance of the first. Each of the two 

 blastomeres divides in a plane at right angles to the preced- 

 ing division. The furrows begin, generally simultaneously, in 

 the upper hemisphere of each of the first two blastomeres, and 

 push toward the lower pole (Fig. 13, A). The upper and 

 lower ends of these new cleavage-planes are sometimes exactly 

 opposite to each other, so that the effect is as though the whole 

 egg had been divided by a single furrow in a plane at right 

 angles to the first. In many cases, however, the new planes of 

 division are not quite opposite, but reach the upper and lower 

 poles of the egg at different points along the first plane 

 of division. A " cross-line " is thus formed. The same re- 

 sult may be brought about even subsequent to division by a 

 shifting or readjustment of the blastomeres on one another. 

 As a rule, when a cross-line occurs in the upper pole, another 

 one is formed in the lower pole, and the two stand in space at 

 right angles to each other, as is shown in the diagrammatic 

 reconstruction in Fig. 14, A. The same result can be obtained 

 by compressing four clay spheres together until a single sphere 

 results. It will be found in such a model that the cross-lines 

 above and below are generally at right angles to each other. 



The third furrows come in at right angles to the preceding 

 planes of division, and are therefore horizontal (Fig. 12, A). 

 The third planes of division do not lie at the equator of the 

 egg, but, taken together, form a small circle in the black hemi- 

 sphere or on the border-line between the black and white areas. 

 Above there are four smaller dark blastomeres and below four 

 larger white blastomeres. The four upper blastomeres are of 

 approximately the same size, but in some species of frogs it 

 seems that one is a little smaller than the rest, one is somewhat 



