284 EMBRYOLOGY. 



sided opening, which is surrounded by five ridges. A knowledge of 

 these is of great importance in studying the history of the formation 

 of the face. 



Of the five ridges one is unpaired, the frontal or naso-frontal 

 process, a broad, rounded projection which bounds the oral pit above. 

 Its origin is connected with the development of the central nervous 

 system, which reaches up to the anterior end of the embryonic 

 fundament, where it is developed into the cerebral vesicles (fig. 153 

 gh, zh, mh). Examined by means of a longitudinal section, the 

 frontal process at this stage, therefore, encloses a large cavity be- 

 longing to the neural tube, and has the form of a vesicle, which is 

 composed of three layers, the epidermis, a layer of mesenchyma, and 

 the thickened epithelial wall of the neural tube. The primary oral 

 cavity and the fundament of the brain are closely apposed at the 

 beginning of development ; they are separated by only a thin sheet 

 of tissue, within whose territory there is subsequently formed, among 

 other things, the floor of the cranium. 



The four remaining ridges are paired structures which surround 

 the oral sinus upon its sides and below. These are produced by 

 growths of the embryonic connective tissue, through which large 

 blood-vessels take their course. They are distinguished according to 

 their positions as upper-jaw (maxillary) and lower-jaw (mandibular) 

 processes. The former are on either side in immediate contact with 

 the frontal process, from which they are separated by a groove only, 

 the naso-optic furrow, which will be discussed in a subsequent chapter, 

 and which runs obliquely upward and outward to that region of the 

 face in which the eye begins its development. The maxillary process 

 is separated from the mandibular process by an incision which corre- 

 sponds to the place of the future angle of the mouth. The two 

 processes of either side together form the pharyngeal arches, or the 

 membranous jaw- arches. 



Before the rupture of the pharyngeal membrane the oral sinus has become 

 still deeper, but only in its upper part, whereas toward the mandibular arch it 

 becomes shallow. This condition is connected with curvatures which in all 

 amniotic Vertebrates as well as Selachians affect that part of the head which 

 encloses the brain-vesicles and lies above the alimentary tube. For the front 

 end of the head is bent down toward the ventral side of the embryo, and 

 finally makes a right angle with the posterior half of the head (fig. 153). 

 Consequently the place at which the so-called anterior cephalic curvature has 

 occurred, and at which the posterior and anterior halves of the head bend into 

 each other, has become an elevation, theparietal [or mid-brain~] elevation (Schei- 

 telhocker), SJf. The latter encloses the middle brain-vesicle (mh), the future 



