THIRD MONTH 



89 



columna nasi (fig. 126). The nose is, however, still broad and flat, while the 

 nostrils remain far apart and look forwards. It is only later that the bridge of 

 the nose is developed, and the nostrils come to be directed downwards. During 

 the second month an epithelial plug develops in each nostril, which occludes the 

 passage for a time. This condition, first described by Kolliker, has recently 

 been further investigated by Retzius. ! There is no doubt that such a plug exists 

 in the foatus figured in fig. 122 (see Development of the Nose). 



The mandibular processes are at first separated by a flat area, but in the sixth 

 week they fuse together, and a mesial projection is developed which is the rudi- 



FIG. 127. SKETCHES SHOWING THE DEVELOPMENT OP THE PABTS OF THB EXTERNAL EAR FROM 

 PROMINENCES UPON THE MANDIBULAR AND HYOID ARCHES. (His.) Variously magnified. 



A, embryo at the end of the first month ; B, embryo of thirty-five days ; C, embryo of thirty-eight 

 days ; D, embryo at the end of the second month. 



1, tuberculum tragicum ; '2, tuberculum anterius helicis ; 3, tuberculum intermedium helicis ; 

 3c and c, cauda helicis ; 4, tuberculum antihelicis ; 5, tuberculum antitragicum ; 6, tuberculum lobulare : 

 L, in A, auditory vesicle ; K, lower jaw. 



ment of the chin. The lips appear as folds of skin in the sixth week, and by the 

 end of the eighth week have considerably advanced in development, but the red 

 margins do not appear till the third month. The eyelids are developed during 

 the same period as the lips, also as folds of skin. The lacrymal groove between 

 the eye and stomodceum is covered over by the fusion of the maxillary and lateral 

 nasal processes. During the second month the branchial arches are telescoped 

 within the hyoid arch, so that the hyomandibular cleft is alone seen on the 

 surface, and by the end of the month the sinus prccervicalis is obliterated. 



Ear. Round the hyomandibular cleft small swellings or tubercles appear, 

 which are named after the different portions of the adult pinna to which they 



1 Biolog. Untersuchungen, 1904. 



