REPEATED DIVISION OF CELLS. 205 



hemisphere parts into a very large number of small cells, 

 the southern hemisphere consists of a much smaller number 

 of larger cells. Finally, they almost entirely overgrow 

 the surface of the spherical egg ; and it is only at a small 

 circular point in the middle of the lower hemisphere, at the 

 south pole, that the inner, larger, and brighter cells are 

 visible. This white space at the southern pole corresponds, 

 as we shall presently see, to the primitive mouth of the 

 Gastrula. The whole mass of inner, larger, and brighter 

 cells (together with this white space at the pole) belongs 

 to trie entoderm, or intestinal layer. The outer envelope of 

 dark, smaller cells forms the exoderm, or skin-layer. 

 ■ The often repeated division of the cells, which as 

 cleavage or segmentation is plainly traceable on the surface 

 of the egg-sphere, is not confined to this surface, but ex- 

 tends to the whole interior of the ball of the egg. The 

 cells also segment in strata, which approximately corre- 

 spond to concentric strata of the sphere ; this process ad- 

 vances more quickly in the upper than in the lower half. 

 A large cavity, filled with liquid forms, has in the mean 

 time arisen, in the interior of the egg-sphere ; this is the 

 cleavage-cavity (s, drawings of sections in Plate II. Fig. 

 8-11). The first trace of this cavity makes its appearance 

 in the middle of the upper hemisphere, at the point at 

 which the three first cleavage-planes, which are at right 

 angles to one another, intersect (Plate II. Fig. 8 s). During 

 the progress of cleavage, this hollow extends significantly, 

 and afterwards assumes an almost hemispherical form (Fig. 

 32 F; Plate II. Fig. 9 s, 10 s). The arched roof of this 

 hemispherical cleavage-cavity is formed by the smaller, 

 barker-coloured cells of the skin-layer, or exoderm (Fig. 



