196 Henry Fox 



groove. It also shows how the antero-internal angle of the sinus praecer- 

 vicalis is continued ventrally into the ventral extension of the second 

 groove. The latter has a decided anterior course, and at its mesial 

 end meets the first groove. 



On the left side the ventral wall of the sinus prsecervicalis is repre- 

 sented as having been removed, so that its interior is clearly shown. 

 The internal process of the sinus is less deep than the same part on 

 the right. 



The continuous transverse fold formed by the ventral diverticula of 

 the first pouch is clearly shown in this view. As in the case of the pig, 

 there is no contact between this fold and the corresponding ventral 

 extension of the groove, the two being separated by a considerable 

 thickness of mesenchyme. 



II. The Later Modifications and Fate of the Pharyngeal 



Pouches. 



Owing to the more or less independent course which the different 

 pouches take in their later history, I think it will conduce to greater 

 clearness if I consider them separately, and accordingly I subdivide 

 the above topic as follows: 



A. The Modifications of the First Pharyngeal Pouch, 

 (a') The Formation of the Primary T}rmpanic Pouch. 



(a") The Differentiation of the Tympanic Cavity and Eustachian 

 Tube. 



B. The Modifications and Fate of the Second Pharyngeal Pouch, 

 (b') The Retrogressive Modifications of the Pouch. 



(b") The Formation of the Tonsillar Fold. 



C. The Metamorphoses of the Third Pharyngeal Pouch and its 

 Derivatives. 



(c') The Elongation of the Ventral Diverticulum and the Forma- 

 tion of the Thymus, 

 (c") The Origin and Structure of the Carotid Gland, 

 (c'") The Sinus Praecervicalis and its Eelation to the Thymus. 



D. The Fourth Phar}Tigeal Pouch and its Transformation into the 

 Tiateral Thyroid and Glandule Thyroidienne. 



A. THE modifications OF THE FIRST PHARYNGEAL POUCH. 



(a') The Formation of the Primary Tympanic Pouch. 

 The pharynx is essentially alike in a pig of 9 mm. (Series M^, my 

 collection) and in a cat of 9.7 mm. (ISTo. 446, Harvard series). Fig. 61 



