148 



SEGMENTATION. 



embryo are not known. During the later stages the segmentation 

 is irregular, and not confined to the surface ; and towards its 



ABC 



FIG. 88. SURFACE VIEWS OF THE EARLY STAGES OF THE SEGMENTATION 



IN A FOWL'S EGG. (After Coste.) 



a. edge of germinal disc; b. vertical furrow; c. small central segment; d. larger 

 peripheral segment. 



close the germinal disc becomes somewhat lenticular in shape ; 

 and is formed of segments, which are smallest in the centre and 

 increase in size to- 

 wards the periphery 

 (figs. 89 and 90). The 

 superficial segments 

 in the 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 segmen- 

 tation reaches its 

 limits in the centre, 

 but continues at the 

 periphery; and thus 

 eventually the masses 

 at the periphery be- 

 come 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 single row of colum- 



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. 



