22 



["CHAP. 



SECTION OF THE GERMINAL Disc OF 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 

 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. 



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 the later stages of segmentation not only do the 

 first-formed segments become further divided, but seg 

 mentation also extends into the remainder of the germi- 

 nal disc. 



The behaviour of the nucleus during the segmenta- 

 tion has not been satisfactorily followed, but there is, 



