42 EARLY EMBRYOLOGY OF THE CHICK 



thickened area is known as the neural (medullary) plate. 

 Laterally the thickened ectoderm of the neural plate blends 

 without abrupt transition into the thinner ectoderm of the 

 general blastodermic surface. Anteriorly the neural plate is 

 more clearly marked than it is posteriorly. At the level of 

 Hensen's node the neural plate diverges into two elongated areas 

 of thickening one on either side of the primitive streak. 



In embryos of 21 or 22 hours (Fig. 14) the neural plate 

 becomes longitudinally folded to establish a trough known as 

 the neural groove. The bottom of the neural groove lies in 

 the mid-dorsal line. Flanking the neural groove on each side 

 is a longitudinal ridge-like elevation involving the lateral por- 

 tion of the neural plate. These two elevations which bound 

 the neural groove laterally are known as the neural folds. The 

 folding of the originally flat neural plate to form a gutter, 

 flanked on either side by parallel ridges, is an expression of the 

 same extremely rapid cell proliferation which first manifested 

 itself in the local thickening of the ectoderm to form the neural 

 plate. The formation of the neural plate and its subsequent 

 folding to form the neural groove are the first indications of the 

 differentiation of the central nervous system. 



The Differentiation of the Embryonal Area. Due to the 

 thickening of the ectoderm to form the neural plate and also 

 to the thickening of the dorsal zones of the mesoderm, the part 

 of the blastoderm immediately surrounding the primitive streak 

 and notochord has become noticeably more dense than that 

 in the peripheral portion of the area pellucida. Because it is 

 the region in which the embryo itself is developed this denser 

 region is known as the embryonal area. Although the embry- 

 onal area is at this early stage directly continuous with the 

 peripheral part of the blastoderm without any definite line of 

 demarcation, they later become folded off from each other. 

 The peripheral portion of the blastoderm is then spoken of as 

 extra-embryonic because it gives rise to structures which are 

 not built into the body of the embryo, although they play a 

 vital part in its nutrition and protection during development. 



The anterior region of the embryonal area is thickened and 

 protrudes above the general surface of the surrounding blasto- 

 derm as a rounded elevation. This prominence marks the 

 region in which the head of the embryo will develop (Fig. 14). 



