THE BLASTODERM. 



411 



have taken place, the germinal area of the egg commences to 

 undergo the process known as segmentation. The bird's egg 

 commences to segment soon after the eggshell is formed. The 

 germinal area, which is a single cell in the ovarian ovum, at 

 first divides into two by means of a furrow running across the 

 germinal disc ; this is shortly followed by another furrow at 

 right angles to it, but not quite symmetrically. Then each 

 of these quadrants becomes cut up by radiating furrows, and 

 each is cut across again in the centre by a cross-furrow, so that 

 the central cells are smaller than the outer ones. This cell- 

 division goes on rapidly, always with greater rapidity towards 

 the centre of the disc. Eventually the whole disc becomes 

 more or less equally divided into a number of small cells, not 

 only from above downwards, but horizontally also. The disc 



Fio. 198 — Sbotion THRonoH PAET OF Blastodebm (first day of incubation). 

 %i, Bpiblast ; Hy, hypoblast. 



now becomes the blastoderm, in which we can distinguish two 

 layers. The cells of the upper layer are closely applied, small, 

 columnar, and with distinct nuclei, and form a definite mem- 

 brane. The lower masses are composed of larger cells some- 

 what rounded and irregularly disposed. This stage may be seen 

 on taking a section of the disc in an egg soon after it is laid. 



From this two-layered blastoderm the bird is built up during 

 the twenty odd days of incubation. Only the area pellucida 

 takes place in the formation of the embryo ; the area opaca 

 gives rise to various appendages of the embryo, which all 

 eventually disappear. The blastoderm, as mentioned above, 

 consists of two layers : the upper layer becomes the epiblast 

 (fig. 198, I^), and the lower the hypoblast (Hy). Between 

 these two a third is formed, the mesoblast. These three ger- 



