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TEXT-BOOK OF EMBRYOLOGY. 



it is not uncommon to speak of all the membranes with the enclosed embryo 

 as the ovum. During the first two months the developing organism itself 

 is usually called an embryo. By the end of the second month when the 

 embryo has reached the length of about an inch (25 mm.) it has acquired a 

 form (Fig. 93) which in general resembles that of the adult and is henceforth 



referred to as afcetus. 



The Face 



When the fore-brain bends ventrally and the heart appears on the ventral 

 side of the embryo in what will be the cervical region, there is thus produced 

 between the two structures a depression or pit called the oral fossa (Fig. 84). 

 This fossa is the rudiment of the oral and nasal cavities and around it the 



Cerebral hemisphere 



Lat. nasal process 



Nasal pit 



Med. nasal process 



Angle of mouth 



Eye 



Naso-optic furrow 

 Maxillary process 

 Mandibular process 



FIG. 96. Ventral view of head of 8 mm. human embryo. His. 



structures develop which- give rise to the face. Behind the fore-brain and 

 dorsal to the heart, as the embryo develops, a series of slit-like depressions 

 appear at right angles to the long axis of the body. Between the depressions 

 are elevations. These structures are in the lateral wall of the embryonic 

 pharynx, and are known as branchial grooves and arches (Figs. 85 and 86). 

 It has been previously stated that they are homologous with the gill slits 

 and gill bars of fishes. The first two arches and the first groove are involved 

 in the formation of the face. 



The first branchial arch becomes the largest of the series and, on ac- 

 count of its position, bounds the oral fossa laterally (Fig, 85). Its presence 

 serves to deepen the fossa. Growing from the cephalic side of the arch, a 

 strong process insinuates itself between the arch and the fore-brain region. 

 This is called the maxillary process, while the original part of the arch is the 

 mandibular process. The latter grows rapidly, extends ventrally and finally 



