The Pbarvngeal Pouches in the Mammalia 217 



Review and Comparisons. 



My investigations make it probable that the history of the second 

 pouch, so far, at least, as its earlier stages are concerned, is similar in 

 the forms studied. Unfortunately, ni}' rabbit and cat material was not 

 sufficiently abundant to enable me to make this statement without 

 qualification. However, the specimens I did examine agreed very closely 

 with corresponding stages in the pig series. The later stages were not 

 sufficiently numerous to enable me to make comparisons. In its general 

 features the development of the tonsillar fold seems to agree in all 

 forms; in details, there are undoubtedly considerable differences in tKb 

 different species. 



The history of the pouch, as mainly determined in the pig, divides 

 itself in two j)eriods — ^the first characterized by a series of retrogressive 

 changes in the pouch, the second by a series of progressive changes 

 converting the remains of the pouch into the tonsillar fold. 



When typically developed the second pouch has the form of a postero- 

 laterally projecting vertical fold. Dorsally the dorso-lateral angle is 

 produced as a dorsal apex. Yentrally it shows a prominent ventral 

 diverticulum. Connection with the ectoderm is more extensive than in 

 any other pouch, the entire lateral margin taking part in the formation 

 of the versclilussmemhran. 



The earlier modifications of the pouch are connected ■with the rapid 

 lateral growth of the hyoid region. The pouch, on the other hand, 

 remains stationary. Parts of it are, however, connected with adjoining 

 structures, and, as these undergo displacement connected with subse- 

 quent growth, the pouch becomes profoundly modified. 



Separation of the pouch from the ectoderm begins on the dorsal side 

 and extends progressively toward the ventral. The last point to remain 

 attached is the ventro-lateral angle of the ventral diverticulum, which 

 becomes drawn out into a thin cord, the filiform process. The latter 

 subsequently separates and then shrinks in length and disappears. 



Largely as a result of the lateral extension of the adjoining tympanic 

 pouch the dorso-anterior portion of the second pouch is drawn farther 

 outwards. Its margin, which originally extended forwards and inwards, 

 acquires an antero-lateral course and thus comes to form a continuum 

 with the posterior border of the tympanic pouch. The underlying 

 antero-lateral surface becomes ventro-lateral and its wall becomes de- 

 pressed to form a deep concavity, which corresponds to the later ton- 



