92 THE DEVELOPMENT OF THE CHICK 
ke S E's growth of the medullary plate, 
<< S Se which causes the latter to extend 
N g & forward above the thinner and more 
-* ce pliable membrane in front. The 
,X & iv entoderm is attached to the inner 
= 
surface of the anterior end of the 
medullary plate (Fig. 47), and is 
apparently carried forward with the 
med. pl. 
\ 
it latter to form the anterior portion 
+ : ; 
<7 of the fore-gut. The actual form of 
Owing to the bending of the primi- 
the fold depends upon the mechani- 
eal properties of the membranes 
NM 
pr. p., Primitive pit. 
pl., Medullary plate. 
x concerned, especially the unequal 
~ +3 ae 
we =e thickness of their parts produced 
; Be ¢~ by unequal growth. 
S SS S ; 
xs CO, 25s Although the head-fold thus ap- 
& 5 : : : ‘ 
Q tees pears to be a single fold involving 
OF eee i 
& z ae the two primary layers, it 1s con- 
Ke © ams = = . . ; J . . at . 
N Sot, venient, for purposes of description, 
io.) 0 a 
Eee. to consider it as two separate folds, 
2A ectodermal and entodermal. The 
deepening of these folds takes place 
at the same rate up to the time 
when four somites are formed (Fig. 
49). At about this time the paired 
primordia of the parietal cavity 
Germ-wall. 
Pr’a., Proamnion. 
(amnio-cardiae vesicles), which ap- 
pear in the mesoblast in the lateral 
W., 
Stage of the first inter 
t. 
tive streak the section passes to one side of the middle line posteriorly. 
extensions of the head-fold (Fig. 
50), push in towards the mid- 
dle line so as to separate the ecto- 
.. Fore-gut. 
dermal and entodermal limbs (Figs. 
52 and 58). When = six somites 
G 
nt., Notochord and entoderm. 
fF, 
are formed, these cavities fuse in 
the middle line, thus effecting a 
complete separation of the two 
limbs. The further progression of 
the head-fold, after this union, 
takes place separately in the two 
Y.S. Ent., Yolk sae entoderm. 
Eetoderm. 
46. — Median sagittal section. 
E 
Bets 
» streak. 
limbs. 
Fic. 
N’ch.4 
tive 
