AVKS. 



259 



st-mik (Fig. 77, 7V), appears. This is a local thickening (Fig. 

 76), where ectoderm and endoderm are continuous, and from 

 which a sheet of mesoderm (mesoblast) grows out on each side, 





Fig. 75. SECTION THROUGH PART OF UNIXCUBATED FOWL'S BLASTODERM 

 (after Klein"). a, Ectoderm ; b, lower layer cells ; f, archenteron ; 

 below this, yolk. 



Fig. 76. TRANSVERSE SECTION THROUGH FRONT END OF PRIMITIVE 

 STREAK IN FIRST DAY CHICK (see Fig. 77) (after Sal/our). Columnar 

 ectoderm above, two cells thick. Pr, Region of primitive streak, 

 where rounded mesoderm cells are originating from the ectoderm ; hy, 

 endoderm, above it, at the side, are stellate mesoderm cells of endo- 

 dermal origin. N.B. Pv is placed over the middle line. 



being reinforced by those lower layer cells which have not 

 united to form endoderm. 



In the anterior part of the blastoderm, i.e., in front of the 

 primitive streak, the lower layer cell form (1) a thin layer 

 of endoderm (hypoblast), (2) two lateral sheets of mesoderm 

 (mesoblast), (3) an axial rod of cells, the notochord, at first 

 closely united with the hypoblast. 



