A VEX. 



123 



iu shape ; and is formed of 

 and increase in size to- 

 wards the periphery (fig. 

 89 and 90). The super- 

 ficial segments in tl. 

 centre of the germinal 

 disc are moreover smaller 

 than those below, and 

 more or less separated as 

 a distinct layer (fig. 90). 

 As development proceeds 

 the segmentation reaches 

 its limits in the centre, 

 but continues at the pe- 

 riphery ; and thus even- 

 tually the masses at the 

 periphery become of the 

 same size as those at the 

 centre. At the time when 

 the ovum is laid (fig. 91) 

 the uppermost layer of 

 segments has given rise 

 to a distinct membrane, 

 the epiblast, formed of a 



segments, which are smallest in the centre 



FIG. 89. SURFACE VIEW OF THE GERMINAL DISC 

 OF FOWL'S EGG DURING A LATE STAGE OF THE SEG- 

 MENTATION. 



c. small central segmentation spheres ; b. larger 

 segments outside these; a. large, imperfectly circum- 

 scribed, marginal segments; e. margin of germinal 

 disc. 





FIG. 90. 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 the 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; 

 if. white yolk. 



