THE FIFTH WEEK. 499 



expressed, the telescoping of the hinder arches within the 

 anterior ones (cf. Fig. 239), has been already referred to. At 

 the end of the fourth week (Fig. 204), the second branchial 

 arch is overlapped by the first, BR.I, and is completely concealed 

 by this in surface views. Early in the fifth week, the second 

 branchial arch is in its turn overlapped by the hyoid arch ; and 

 from about the thirtieth day onwards (Fig. 205) the only 

 arches visible on the surface of the neck are the maxillary, 

 mandibular, and hyoidean. Behind the hyoidean arch is the 

 deep fissure caused by the sinus preecervicalis (cf. Fig. 240, su), 

 which must not be mistaken for a visceral cleft. 



During the fifth week the borders of the hyomandibular cleft 

 become more prominent, and gradually give rise to the folds 

 from which the external ear is developed, in the following manner. 



At the end of the fourth week (Fig. 204), the hyomandi- 

 bular cleft, HM, is a deep groove between the mandibular and 

 hyoid arches, and running about halfway across the head. 

 The mandibular arch is divided by a slight constriction, about 

 the middle of its length, into dorsal and ventral portions : of 

 these, the ventral portion bears at its upper and posterior border 

 a small rounded process, well shown in the figure, and named 

 the tuberculum tragicum ; while the dorsal portion of the arch, 

 to which the reference line, MN, runs, is the tuberculum anterius 

 helicis. Opposite the dorsal end of the hyomandibular cleft is a 

 longitudinal ridge, the tuberculum intermedium helicis. 



The hyoid arch is divided, by two transverse constrictions, 

 into dorsal, middle, and ventral lobes : of these, the dorsal lobe is 

 named the tuberculum anthelicis ; the middle lobe, to which the 

 reference line, HY, in Fig. 204 runs, is the tuberculum anti- 

 tragicum ; and the ventral lobe, which is the smallest of the 

 three, is the tuberculum lobulare. 



In the course of the fifth week, these swellings assume more 

 definite form, and gradually give rise to the several parts of the 

 external ear or pinna. The tuberculum anterius helicis (Fig. 

 208, 2), and tuberculum intermedium (3) unite together, and 

 with a vertical ridge, the cauda (sc), which arises along the 

 posterior border of the hyoid arch, to form the horse-shoe shaped 

 marginal rim, or helix, of the ear. The ventral ends of the hyoid 

 and mandibular arches fuse, and so give more definite shape to the 

 hyomandibular cleft, which latter becomes the external auditory 



K K 2 



