CLEAVAGE AND FORMATION OF THE GERM-LAYERS. 



221 



VI. 



the egg (Fig. 106 A). Only after this has taken place do the 

 separate cell-areas become marked off from one another (Fig. 106 B), 

 and thus produce the appearance of total cleavage mentioned above. 

 We may assume that some of the central nuclei had already shifted 

 towards the periphery, and that the cleavage of the egg is a con- 

 sequence of this. It is probable that the yolk -pyramids which 

 are bounded by the furrows are provided with nuclei, although it 

 has been impossible up to the present time to prove this with 

 certaintj'. Sograff assumes that the nuclei belonging to the pyra- 

 mids lie at their tips, and are therefore not far removed from the 

 central nuclei. The yolk-pyramids are not completely marked off 

 from one another, but are connected at the centre of the egg where 

 the central nuclei lie (Fig. 106 B), 



After the cleavage of the yolk has taken place, a migration of the 

 central nuclei towards the periphery occurs. The nuclei force their 

 way into the yolk-pyramids, 

 the number of which has 

 increased, and shift towards 

 the periphery of the egg 

 (Fig. 107). Judging from 

 Sograff's figures, this migra- 

 tion seems to take place chiefly 

 along the boundaries of the 

 yolk-pyramids (Fig. 107 .4). 

 Eeachino- the surface of the 

 egg, the nuclei at first are 

 not evenly distributed, but 

 arranged in groups (Metsch- 

 nikoff, Sograff, Heathcote), 

 but later they form a con- 

 tinuous layer of cells, the 

 blastoderm. The latter de- 

 velops first on the ventral 

 surface (Fig. 107 B), where the cells divided more quickly, and 

 consequently become smaller, and proceeds from this to the dorsal 

 surface, where, until now, the cells were still arranged in groups 

 (Fig. 107 B, gr). The yolk-pyramids remain distinct until some 

 time after the formation of the blastoderm. 



Most authors agree in assuming that in the formation of the 

 blastoderm a large proportion of the nuclei remain within the egg, 

 perhaps in the yolk-pyramids. This cell-material represents for the 



Fig. 108.— Section through an embryo of Julus 

 tcrrcstris on the sixteenth clay of development, 

 somewhat diagrammatic (after Heathcote). 

 hi, blastoderm ; d, yolk ; do, dorsal surface ; 

 dz, yolk-cells ; k, the keel-shaped accumulation 

 of cells on the ventral side (ue). 



