CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 



245 



equatorial furrow. The two narrow posterior segments, so character- 

 istic in shape and position, still remain unaltered even after this 

 division, but two further blastomeres gradually become detached 

 from them by equatorial division and pressed towards the centre 

 (Fig. 109 0)." - 



The number of segments increases more and more through the 

 appearance of other furrows, some meridional and others equatorial 

 (Figs. 108 C, 110). From the accounts of KOLLIKER and V LILLE- 

 TON, it appears that the segments at the middle of the germ-disc are 

 not at first in close contact with each other (Figs. 107 and 109). As 

 cleavage advances, this space in the centre disappears. 



JT. 



FIG. 110. Germ-disc of /,// Pealii at a later stage of cleavage, the bilateral sym- 

 metry still being evident. The blastomeres and blastocones lie symmetrically to the 

 middle line /-/ (after WATASE). ro, anterior; h, posterior; I, Jeft ; r, right; I-V, 

 direction of the first fi ve furrows. 



The bilateral symmetry evident up to this time and still visible in some- 

 what later stages is still further heightened by the division of the cells taking 

 place at somewhat different times, a fact which finds expression in the 

 different conditions of their nuclei. Such a case is represented, for instance, 

 in Fig. 110, in which the posterior cells lying near the middle line contain 

 resting nuclei while the nuclei of all the other cells are found to be dividing. 

 This phenomenon is also frequent in the larger cell-complexes. The almost 

 diagrammatic aspect thus produced corresponds, as WATASE expressly states, 



