HEAD-FOLD TO TWELVE SOMITES 95 
Another place of fusion between the fore-gut and the ecto- 
derm is the so-called oral plate (pharyngeal membrane), which 
occupies a mid-ventral position at the extreme anterior end. 
The parietal cavities meet posterior to the oral plate (Figs. 67 
and 75). Transverse sections show the oral plate to be depressed 
beneath the level of the ventral surface of the head at the stage of 
10 somites (Fig. 55), a condition that increases, as development 
g 
‘ HID 
<f ENCE sie eam 
IG RABE aAT a7 
O'S) > 
=, — 
Oxy L£3R> “TES 
Fig. 49. — Median sagittal section of the head at the stage of 4s. 
a. i. p., Anterior intestinal portal. F.G., Fore-gut. Ect., Eetoderm. 
Ent., Entoderm. H.F., head-fold. Mes., Mesoblast. n. F., Neural fold. 
or. pl., Oral plate. 
proceeds, by the formation of the cranial flexture, and by the up- 
growth of the tissues behind and at its sides; thus will be estab- 
lished a deep depression lined by ectoderm, the floor of which is 
formed by the oral plate, and which is destined to form a large 
part of the mouth. The depression is known as the stomodeum. 
III. Oricin or THE NEURAL TUBE 
The Medullary Plate. The medullary plate is the primordium 
of the central nervous system. At the time of formation of the 
head-fold it is broad in front and narrower posteriorly, ending 
opposite the posterior end of the primitive streak. Its central 
portion is not a separate plate of cells in the region of the primi- 
