130 DEVELOPMENT OF THE AURICLE IN THE HUMAN EMBRYO. 



the drawing was made. I may mention at this point that all of these drawings 

 were made directly from the specimens by Mr. J. F. Didusch. In most of them the 

 embryo was stained slightly in order to define more clearly the surface markings. 



In stage K (cf. fig. 23) the period of branchial hillocks may be regarded as 

 having passed. The remnants of hillocks 1, 2, and 6' can still be recognized; other- 

 wise, the borders of the fossa angularis are now made up of the sloping surface of 

 the crus helicis and the primitive ear-fold or scapha-helix. Microscopic examina- 

 tion of a transverse section through the ear-fold at this time shows it to be due to a 

 mass of condensed mesenchyme, although differing from the condition found during 

 the hillock period in that there is now a precartilaginous outline of the auricular 

 cartilage, the contours of which can be made out along the posterior edge of the 

 condensed tissue. From the outset this precartilage assumes the typical outlines 

 of the auricular cartilage. 



On coming to stage L (cf. fig. 24) we can speak only of remnants of hillocks 1 

 and 6'. The crus helicis is becoming more distinct and the primitive ear-fold more 

 prominent. With the formation of the crus helicis the fossa angularis loses its 

 identity, and in its stead there is the early form of the concha, divided by the crus 

 into an upper and a lower half. 



Stage M (cf. fig. 25) shows a rather marked primitive ear-fold, which is prob- 

 ably a peculiarity of this particular specimen. It may be assumed that any extreme 

 characteristics of the adult ear would have begun to express themselves at this 

 time, and it may be that in this case we would have had an ear with a prominent 

 tip. The tendency toward a pointed process of the ear-fold, however, is an artifact 

 of preservation. 



The specimen used to illustrate stage N (cf. fig. 26) is somewhat fuller than the 

 preceding specimen and is more characteristic. The transition from stage N to 

 stage (cf. fig. 27) brings us to a condition that may be regarded as the definitive 

 auricle. We can now recognize the tragus, antitragus, anthelix, scapha-helix, and, 

 distinctly separate from the latter, the crus helicis. In tracing the hillocks up to 

 this point, it is found that the only ones that can be said to persist are hillock 1 

 (as the tragus) and hillock 6' as the antitragus. All of the others lose their identity 

 in the transition of the tissues forming the margins of the angular fossa into the 

 definitive auricle. Sections through the auricle at this time disclose the fact that 

 the condensed mesenchyme, which heretofore made up these elevations, is now 

 entirely resolved into the cartilaginous plate representing the auricular cartilage 

 and the looser subcutaneous tissues, including the muscles and ligaments of the 



ELABORATION OF THE AURICLE. 



On plates 4, 5, and 6 I have arranged a series of photographs showing the auricle 

 at different stages of fetal development. It is thus possible to trace the develop- 

 ment of its different parts by following them through these photographs. The 

 increase in the size of the auricle holds only for the individual plate, the photo- 

 graphs on plate 4 being enlarged 10 diameters, those on plate 5 being enlarged 6 

 diameters, and those on plate 6 being enlarged 4 diameters. In studying them, one 



