AYES. 257 



in the lower part of the oviduct, and by a succession of furrows, 

 some at right angles and others parallel to the surface, the 

 germinal disc is converted into a many-celled blastoderm. This 

 is placed on one side (Fig. 73, Bl), and always remains upper- 

 most, the most favourable position for development, which 

 requires a good deal of warmth, supplied in nature by the 

 body of the hen. Before incubation (Fig. 75) the blastoderm 

 consists of a superficial layer, one-cell thick, of columnar ecto- 



Fig. 73. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH UNINCCT BATED 

 HK.N'.S EGG (from Claus, after Balfour and Allen Thomson). Bl, 

 Blastoderm; GD, yellow yolk; WD, white yolk; DM, vitelline 

 membrane; EW, "white;" Ch, chalazae ; S, shell membrane; K8, 

 shell ; LR, air-chamber. 



derm (epi blast), and a more irregular thickened mass of lower 

 layer celk, which are rounded, granular, and of larger size. Below 

 them is a space, the earliest rudiment of the alimentary canal. 



Previous to laying, a blastOCCBle (segmentation cavity) can be seen 

 between ectoderm and lower layer cells. 



The blastoderm is at first circular, and the more transparent 

 central part, known as the area pellucida, is surrounded by a 

 darker rim, the area opaca (cf. Fig. 77). If the egg is placed 

 with its broad end to the right, the diameter of the blastoderm, 

 which is to become the long axis of the embryo, will run across 

 the egg, the posterior end being towards the observer. 



It now remains to describe the formation of the endoderm 

 and mesoderm. These are first clearly differentiated in the 

 2 17 



