24 



THE HEN S EGG. 



[chap. 



important to note that the central segments divide more 

 rapidly than the peripheral, and consequently become at once 

 smaller and more numerous (Fig. 6). 



Meanwhile sections of the hardened blastoderm teach us 

 that segmentation is not confined to the surface, 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. 7). 



Fig. 7. 



Section of the Germinal Disc op a Fowl during the Later Stages 



OF Segmentation. 

 The section, which represents rather more than half the breadth of the 

 blastoderm (the middle line being shewn at c), shews that the upper and central 

 parts of the disc segment faster than those below and towards the periphery. 

 At the periphery the segments are still very large. One of the larger segments 

 is shewn at a. In the majority of segments a nucleus can be seen ; and it seems 

 probable that a nucleus is present in them all. Most of the segments are filled 

 with highly refracting spherules, but these are more numerous in some cells 

 (especially the larger cells near the yolk) than in others. In the central part of 

 the blastoderm the upper cells have commenced to form a distinct layer. No 

 segmentation-cavity is present. 

 a. large peripheral cell. b. larger cells of the lower parts of the blastoderm. 



c. middle line of blastoderm, e. edge of the blastoderm adjoining the white 



yolk. w. white yolk. 



In this way, by repeated division, or segmentation, the 

 original germinal disc is cut up into a large number of small 

 rounded masses of protoplasm, which are smallest in the 

 centre, and increase in size towards the periphery. The 

 segments lying uppermost are moreover smaller than those 

 beneath, and thus the establi.shment of the two layers of 

 the blastoderm is foreshadowed. 



