122 SEGMENTATION. 



Owing to the absence of symmetry the cells at one side of the ger- 



e/i.l 



Fig. 87. Diagbammatic section of an unincubated Fowl's egg. 

 (Modified from Allen Thomson.) 

 bl. blastoderm; w.y. white yolk. This consists of a central flask-shaped mass and 

 a number of layers concentrically arranged around it. y.ij. yellow yolk; v.t. vitel- 

 line membrane; x. layer of more fluid albumen immediately surrounding the yolk; 

 w. albumen consisting of alternate denser and more fluid layers; ch.l. chalaza; axh. 

 air-chamber at the broad end of the egg. This chamber is merely a space left 

 between the two layers of the shell-membrane, i.s.m. internal layer of sheU-membrane ; 

 s.m. external layer of shell-membrane; s. shell. 



minal disc are larger than those at the other, but the relations between 



Fig. 88. Surface views op the early stages of the segmentation in a Fowl's egg. 



(After Coste.) 

 a. edge of germinal disc; 6. vertical furrow; c. small central segment; d. larger 

 peripheral segment. 



the disc and the axis of the embryo are not known. During the later 

 stages the segmentation is irregular, and not confined to the surface ; 

 and towards its close the germinal disc becomes somewhat lenticular 



