416 Nose, Phaiyux and its Derivatives in Man 



the 2nd week, to an angular, much differentiated cavity, during the 

 4th week. After that time it gradually loses its angularity, until it is 

 a curved and well rounded cavity in the embryo of the 7th week. 



The angle at the aboral end of the pharynx gradually shifts its posi- 

 tion from a point opposite the 3rd visceral pouch in an embryo of the 

 3rd week, where it is most pronounced, until it is just oral to the 2nd 

 visceral arch. As an angle it has completely disappeared in an embryo 

 of the early part of the Gth week. 



The Eustachian tube is the result of an extension of the cephalic 

 angle and ridge of the 1st visceral pouch and the narrowing of the 

 buccal cavity. 



The sulcus tubo-tympanicus of Moldenhauer is a cephalic extension 

 of the groove running toward the middle line from the 1st pharyngeal 

 pouch. A part of it is converted into the mesial portion of the Eus- 

 tachian tube by the conversion of the sulcus into a lube through the 

 gradual narrowing of the buccal cavity. 



The relative position of the posterior nares changes by gradually 

 moving backward and becoming larger. The posterior nares and the 

 Eustachian tube in the embryos of the first few weeks are widely 

 separated bat this shifting of the position of the posterior nares brings 

 them into the relative positions which they occupy in the adult. 



The turbinate processes show themselves as elevations quite early in 

 the Gth week. The inferior is the most distinct. These gradually 

 undergo differentiation and separation, but even in the 7th week they 

 are still much unlike the adult condition. 



In man the thyroid arises from the union of a medifai rudiment situ- 

 ated at the point of junction of the tuberculum impar and the two 

 dorsal rudiments of the tongue with a paired rudiment arising as a 

 differentiation of the lining of the 4th visceral pouch. 

 ' In man the rudiment of the thymus arises from the endothelium of 

 the 3rd visceral pouch. 



In the development of the pharynx the left side develops more 

 rapidly than the right side, thus causing a lack of bilateral symmetry. 

 This difference gradually decreases until by the 7th week the two sides 

 have become about equalized. The cause of this difference is unknown 

 biit probably the bending c;f the head, as Mall suggests, may have some 

 influence in determining it. 



Note. — The original drawings to illustrate this article were made by 

 My. George T. Kline and the reproduction of them superintended by 

 Mr. F. S. Lockwood. I am greatly indebted to them for their care and 

 interest in the work throusfhout. 



