DEVELOPMENT OF THE AURICLE IN THE HUMAN EMBRYO. 127 



branchial hillocks. The significance of the subdivision of these bars has never been 

 determined ; we shall see, however, that the closure mechanism is derived from the 

 ventral portions, while the articular and sound-collecting mechanisms are derived 

 from the dorsal portions. His (1882), in describing the mandibular bar in young 

 embryos, mentions the existence of a root part (Wurzelstuck) as distinguished from 

 the more ventral portion, which he describes as divided longitudinally into a lip 

 ridge (Lippenwulst) and a mental ridge (Kinnwulst). Careful examination of 

 figures 10 and 11 (plate 1) will show that the ventral part of the mandibular arch 

 is roughly subdivided into two ridges, somewhat as described by His. These 

 ridges do not, however, correspond exactly to the eventual chin and lip, as His 

 first thought. The more anterior one (lip-ridge) in reality gives origin to the greater 

 part of the jaw, the lip being a much later derivative of it. The more posterior 

 ridge (Kinnwulst) corresponds to the soft parts beneath the jaw. 



The origin of the branchial hillocks and their fate are shown in figure 5. This 

 figure is intended as a diagrammatic interpretation of figures 13 to 27 (plates 2 and 

 3). For convenience I have lettered these as a series of successive stages. By 

 comparing them it will be seen that definite hillocks make their appearance in 

 embryos about 10 mm. long, reach their full development in embryos about 14 mm. 

 long, and disappear for the most part between 16 and 18 mm. At stage B, when 

 they first appear (cf. fig. 14), one finds on the dorsal segment of the hyoid bar two 

 opaque elevations corresponding to hillocks 4 and 5. Hillock 4 is strongly sug- 

 gestive of a facial placode, but on tracing it into the succeeding stages (figs. 15 to 

 18) it becomes evident that this can not be the explanation. On the ventral seg- 

 ment of the hyoid bar in stage B can be seen an opaque thickening repre- 

 senting the first appearance of hillock 6. At stage C (cf. fig. 15) the three hyoid 

 hillocks are clearly indicated, and at the same time the first indication of hillock 1 

 can be recognized on the ventral segment of the mandibular bar. The dorsal 

 segment of this bar still forms a round mass corresponding to the Wurzelstuck of His. 



At stage D (cf. fig. 16) hillocks 4 and 5 are sharply rounded and have reached 

 their maximum development. Hillock 6 becomes subdivided, as indicated in the 

 diagram, and, as will be seen, it is hillock 6' that eventually forms the antitragus. 

 At this time the dorsal part of the mandibular bar shows the first evidences of 

 hillocks 2 and 3. Along with the appearance of these hillocks the hyoid cleft is 

 widened to form a definite fossa, the fossa angularis of His. 



Stage E (cf. fig. 17) represents the hillocks at their maximum development, 

 and it is their appearance at this time that led to the classical description of His 

 and to the numbering of the hillocks serially 1 to 6. Furthermore, it is this appear- 

 ance that we find duplicated in embryos of other mammals and which resembles 

 also the condition found in birds and reptiles. Microscopic examination of sections 

 through the hillocks at this time shows that they consist of rather sharply outlined 

 masses of condensed mesenchyme cells closely packed against the covering ecto- 

 derm. The ectoderm itself is in active proliferation and is much thicker than that 

 of the surrounding regions. In embryos about 11 mm. long the ectoderm can be 

 seen to consist of two layers a more superficial, flattened membrane one cell 



