THE GERMINAL LAYERS. 361 



the broader end of the embryonal area may be called the 

 anterior end, and the narrower one the posterior end. 



As regards the structure of the embryonal area, Rauber's 

 layer has disappeared almost completely ; a few individual cells 

 may still be recognised here and there, but there is no longer a 

 continuous stratum of cells. In consequence of the disap- 

 pearance of Rauber's layer (cf. Fig. 141), the embryonal area 

 now consists of only two layers of cells : (i) the epiblast, or 

 former middle layer, which now becomes the superficial layer, 

 consists, as before, of a single layer of short columnar cells ; it 

 thins towards the margin of the embryonal area, and at its 

 margin is said to become continuous with the outer layer of 

 cells, or epiblast cells of the rest of the blastodermic vesicle : 

 (ii) the hypoblast, in the embryonal area, has the same characters 

 as before ; beyond the embryonal area, it has now extended about 

 half way round the inner surface of the blastodermic vesicle. 



The blastodermic vesicle at the end of the seventh day is, 

 therefore, an ellipsoidal sac filled with fluid. Its wall consists, 

 in the upper half of the vesicle, of two layers of cells, epiblast 

 and hypoblast ; in the lower half, of a single layer, the epiblast 

 alone. In the middle of the upper half of the vesicle is the 

 embryonal area, which is also two-layered, but in which the 

 epiblast differs from that of the rest of the vesicle in consisting 

 of columnar instead of pavement cells, 



4. The Primitive Streak and the Mesoblast. 



Towards the close of the seventh day, the primitive streak 

 appears. This structure, which in the mode of its formation, 

 and in its relations to other parts, agrees closely with that of the 

 chick, is at first an axial thickening of the epiblast at the 

 posterior, or narrower, end of the embryonal area. It rapidly 

 lengthens, and by the end of the seventh day (Fig. 143, PS) 

 it extends, as a linear opacity, along about two-thirds of the 

 length of the area, having a faint longitudinal groove, the primi- 

 tive groove, along its dorsal surface. 



Transverse sections at this stage (Fig. 142, rs) show that 

 the primitive streak is formed by proliferation of cells from the 

 under surface of the epiblast, in the median plane. 



The mesoblast. The cells of the primitive streak spread 

 out, beyond the margins of the thickened streak itself, as two 



