xm DEVELOPMENT OF CHICK 583 



The first stage of segmentation is indicated by the forma- 

 tion of a vertical furrow across the centre of the germinal 

 disc (Fig. 153, A), and this is followed by another furrow at 

 right angles to it (B) . Further radial and concentric furrows 

 then arise (C) until the blastoderm is seen to consist of 

 a number of irregular cells (D). Horizontal furrows are in 

 the meantime formed, so that the originally single layered 

 blastoderm, beneath which a narrow space representing the 

 segmentation-cavity can be recognised, comes to consist of 

 several layers of cells, the outer of which corresponds to the 

 ectoderm ; the inner, or lower layer cells, extend rapidly 

 through the segmentation-cavity between the ectoderm and 

 the yolk and give rise to the endoderm, the space remaining 

 beneath this layer and the yolk forming the rudiment of the 

 enteron (F). New cells are gradually formed round the edge 

 of the blastoderm, which, when the egg is laid, forms a 

 circular patch, about 3-5 mm. in diameter, on the surface of 

 the yolk (Fig. 149) : its central part, or area pellucida, owing 

 to the presence of fluid in the underlying space, is more 

 transparent than the rim, or area opaca. 



\Yhen the egg is incubated, the blastoderm grows rapidly 

 at its periphery (area opaca) . and eventually, as we shall see, 

 encloses the whole yolk (Fig. 159). The area pellucida 

 extends less rapidly and becomes pear-shaped in outline 

 (Fig. 153, E), its broader end corresponding to what will 

 become the anterior end of the embryo : in this region most 

 of the lower layer cells are arranged so as to form a definite 

 layer of endoderm, which can be recognised somewhat later 

 in the area opaca also. The ectoderm and lower layer cells 

 are at first continuous round the edge of the blastoderm, 

 but later on this is only the case at the posterior border (F), 

 and results in the appearance of a narrow band, the primitive 

 streak (E, pr. st), along which a median primitive groove is 

 formed. This, as already mentioned, represents the fused 

 lips of the blastopore of the frog and other forms ; it extends 

 from the centre to the posterior border of the area pellucida, 

 and rapidly grows backwards. The primitive streak is due 

 to the multiplication of ectoderm cells, which grow inwards 

 and spread out right and left under the covering ectoderm, 

 between it and the endoderm, so as to form a horizontal 

 wing of cells on either side (G). The mesoderm is formed 

 from these wing-like ectodermic sheets, with which the endo- 

 derm becomes united, so that the three layers are for a time 

 indistinguishable in this region. The mesoderm eventually 

 forms a sheet of loosely arranged cells which spreads rapidly 

 in all directions except'anteriorly (El , in which region, known 

 as the pro-amnion, the blastoderm consists of ectoderm and 



