DEVELOPMENT OF THE AURICLE IN THE HUMAN EMBRYO. 129 



The branchial hillocks never reach the same degree of development on the 

 mandibular arch that they do on the hyoid arch, making their appearance later and 

 disappearing earlier. At stage F (cf. fig. 18) they can still be recognized, although 

 they are less distinct than they were in the preceding stage. The specimen selected 

 to illustrate this stage exhibits an anomaly of the fossa angularis, in that a relatively 

 large ridge appears to extend from the region of hillock 2. This is not to be mis- 

 taken for the cms helicis; it is due apparently to some peculiarity of this specimen. 

 Hillocks 6 and 6' are very characteristic at this stage. The latter curves inward 

 and forward, forming a different plane from hillock 6. The two are still, however, 

 partially connected. 



At stage G (cf. fig. 19) hillock 3 is beginning to disappear, and hillock 2 

 is crowded to a more ventral point by the change that has taken place in that 

 region of the mandibular bar, which is preliminary to the formation of the crus 

 helicis. The fossa angularis now forms a rather roomy quadrilateral depression 

 whose floor in this and the next succeeding stages bulges out slightly, correspond- 

 ing to the development of the tissues in the neighborhood of the head of Meckel's 

 cartilage. 



At stage H (cf. fig. 20) hillocks 1, 2, 6, and 6' are still clearly defined. Hillock 

 5 can still be recognized but is becoming less distinct. Hillocks 3 and 4 can scarcely 

 be outlined, but in then place is a ridge which forms the rounded contour of the 

 upper end of the fossa angularis. The tissue lying under the other hillocks has been 

 constantly increasing in amount, having the effect of increasing the depth of the 

 fossa. 



At stage I (cf. fig. 21), coincidentally with the gradual disappearance of the 

 hillocks, the raised margin of the fossa angularis begins to take the form of definitive 

 parts of the auricle. One can see in the region formerly occupied by hillock 3 that 

 the first evidence of the crus helicis is making its appearance. In the region cor- 

 responding to hillock 4 the upper end of the fold in which will form the helix can 

 readily be recognized. The last traces, however, of hillocks 5 and 6 are to be seen. 

 Hillocks 1 and 2 are still quite definite. The relative sizes of hillocks 1 and 2 appear 

 to vary, as does also the degree of separation between them. 



At stage J (cf. fig. 22) the conditions are much the same as in the preceding 

 specimen, although the fold of the helix appears to be a little more pronounced 

 and the last vestige of hillock 5 has disappeared. 



In studying these hillocks I find that the angle from which they are viewed 

 and the method of illumination have a great deal to do with their appearance. 

 It has also proved necessary to make considerable allowance for the condition of 

 the tissues and the manner of fixation. In the specimens selected for illustration 

 I have attempted to include only the normal and average ones, but even with this 

 precaution I am conscious of the possibility of having introduced examples that 

 are not necessarily typical. I am somewhat doubtful regarding figure 22, as well 

 as figure 18, the peculiarity of which has already been mentioned. In figure 22 the 

 thick fossa angularis is somewhat exaggerated, as is also the fold of the helix. The 

 embryo is absolutely normal, but the tissues seemed a little shrunken at the time 



