PHARYNX AND AORTIC ARCHES 409 



Before the various pharyngeal derivatives develop, there are 

 several changes in the general relations of the pouches. The 

 second, third, and fourth clefts come to end in a deep cervical 

 sinus, into which the second and third pouches open and to 

 which the fourth is joined by a solid stalk (fig. 8). The whole 

 embryo grows in such a way that the pouches are first 

 pulled out into long tubes. Next, the clefts are deepened into 

 tubular prolongations of the pouches and the placodes are 

 carried from the external body surface into the recesses of the 

 clefts. Finally the external apertures of the second and fourth 

 clefts are drawn towards that of the third, and all three then 

 are buried in a common cervical sinus. 



At the same time the pharyngeal wall between the fourth 

 pouch and the postbranchial body is pulled out into a common 

 stalk for both (fig. 7). The fourth pouch is much the larger. 

 Whe» the common stalk is completed, it appears to be part of 

 the fourth pouch, to which the postbranchial body then seems 

 to be attached (fig. 8). 



The fifth pouch meanwhile degenerates rapidly; it is repre- 

 sented in the 9.5-mm. embryo (fig. 8) by a slight heap of cells 

 placed at the union of the postbranchial body and the fourth 

 pouch. In older embryos it is lost altogether. 



In the following paragraphs, the several pouches and other 

 pharyngeal structures will be considered in- further detail, each 

 in turn. 



First and second pouches. The later development of the 

 first two pouches can be passed over briefly. From the first 

 there develops, in a 9.5-mm. embryo, a tympanic process. At 

 10.6 mm., this process pushes forward beneath the columella 

 and then turns back over its dorsal surface. The tip of the 

 process expands into a large irregular sac, which fills the tym- 

 panic and mastoid cavities. The proximal end of the tympanic 

 process and the remnant of the first pouch are converted into 

 a relatively large auditory tube, to which the second pouch may 

 also contribute. The method of development of the tympanic 

 cavity is very similar to that of Chelydra, which has been care- 

 fully described and excellently illustrated by Dohrer. 



