CHAPTER V 
HEAD-FOLD TO TWELVE SOMITES 
(From about the twenty-first to the thirty-third hour of incubation) 
I. ORIGIN OF THE HEAD-FOLD 
At the end of the period described in Chapter IV, the embryo 
is represented by a central differentiated area of the blastoderm, 
lying within the area pellucida, distinguished anteriorly by the 
medullary plate and head-process, and posteriorly by the primitive 
streak. The layers of the embryonic area are everywhere continu- 
ous with the corresponding layers of the extra-embryonic blasto- 
derm, with no clear line of division between the two. In the course 
of the second and third days the embryo becomes clearly defined 
by its own growth, and by the formation of bounding folds. 
The delimitation of the embryo from the blastoderm begins 
immediately after the formation of the head-process by the for- 
mation of a fold at the anterior end of medullary plate known as 
the head-fold (Fig. 42). Seen from the surface, this fold has a 
semicircular outline, the concavity of which is directed posteriorly 
(Fig. 44). It involves both the ectoderm and entoderm. A later 
stage is shown in sagittal section in Figs. 46 and 47: the ecto- 
derm and entoderm immediately in front of the medullary plate 
make a sharp bend downwards and backwards, and then turn 
forward again. The head-fold thus produces an internal bay in 
the entoderm, the beginning of the fore-gut. There is similarly an 
external bay, the posterior angle of which is the head-fold proper, 
lying beneath the projecting head. These bays are of course 
turned in opposite directions, the internal one opening into the 
subgerminal cavity posteriorly, and the external one opening 
anteriorly on the surface of the blastoderm. 
The transition from the ectoderm of the medullary plate into 
that of the under surface of the head and the proamnion is a grad- 
ual one. The difference is, however, very strongly marked (Tig. 
47). The formation of the head-fold is due to the more rapid 
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