Development of the First Day 123 



The differentiation of the entoblast begins 

 in the centre of the area pellucida, and gradu- 

 ally extends towards the area opaca, where 

 it is formed at a somewhat later period, and 

 where it is composed of cells of a different 

 shape from those of the pellucid area. 



While the entoblast is being formed, the 

 blastoderm has increased considerably in size, 

 and the area pellucida, which in the unincu- 

 bated egg is often very indistinct, becomes 

 sharply marked off from the opaque area. 



About the middle, or during the second 

 half, of the first day, a very characteristic 

 structure, the primitive streak, makes its ap- 

 pearance. With the naked eye, or under a low 

 power of the microscope, it is seen (Fig. 40, X) 

 as a distinct linear opacity in the posterior half 

 of the now pear-shaped area pellucida. 



It might here be mentioned that the broad 

 end of the area pellucida corresponds to the 

 head end of the future chick ; and that if 

 the egg be held with its large end towards 

 the right, in nearly every case the head of the 

 embryo will point away from the observer. 



If vertical sections be cut through the primi- 

 tive streak, at right angles to its long axis 

 (Fig. 41, pp), it will be found that the streak 



