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 



FIG. 49. Median sagittal section of the head at the stage of 4 s. 



a. i. p., Anterior intestinal portal. F. G., Fore-gut. Ect., Ectoderm. 

 Ent., Entoderm. H. F., head-fold. Mes., Mesoblast. n. F., Neural fold, 

 or. pi., 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 stomodseum. 



III. ORIGIN OF 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- 



