20 THE hen's egg. [chap. 



Tlie second furrow cuts the first somewhat excen- 

 trically. 



The first four furrows do not extend through the 

 whole thickness of the germinal disc, and the four seg- 

 ments marked out by them are not separated from the 

 disc on their lower aspect. 



Each of these is again bisected by radiating furrows, 

 and thus the number of segments is increased from four 

 to eight (it may be seven or nine). The central portion 

 of each segment is then, by a cross furrow, cut off from 

 the peripheral portion, giving rise to the appearance of 

 a number of central smaller segments, surrounded by 

 more external elongated segments (Fig. 6, G). 



The excentricity in the arrangement of the segments 

 is moreover still preserved, the smaller segments being 

 situated nearer one side of the germinal disc. The 

 excentricity of the segmentation gives to the segmenting 

 germinal disc a bilateral symmetry, but the relation 

 between the axis of symmetry of the segmenting germinal 

 disc and the long axis of the embryo is not known. 



Division of the segments now proceeds rapidly by 

 means of furrows running in various directions. And 

 it is important to note that the central segments 

 divide more rapidly than the peripheral, and con- 

 sequently become at once smaller and more numerous 

 (Fig. 7). 



Meanwhile sections of the hardened blastoderm 

 teach us that segmentation is not confined to the sur- 

 face, but extends through the mass of the blastoderm ; 

 they shew us moreover that division takes place by 

 means of not only vertical, but also horizontal furrows, 

 ■i. e. furrows parallel to the surface of the disc (Fig. 8). 



