216 Henry Fox 



In form the tonsillar fold (Ton.F.) of the pig of this stage hears a 

 greater resemblance to that of the cat than to the same structure in 

 the rabbit. The tonsillar fold of the latter has a more decided ascend- 

 ing plane than the others. 



In the cat of 23.1 mm. (N"o. 466, Harvard series, Figs. 66-67) the 

 tonsillar fold (Ton.F.) has attained its definite position. Tlie palatal 

 constriction has now separated the nasal cavity from the mouth and 

 has begun to encroach upon the pharynx. The tonsillar fold forms a 

 wide, diagonally ascending arched fold on the side of the oral portion. 

 Its ventro-lateral surface is, as usual, deeply concave. 



The pig of 32 mm. (No. 74, Harvard series, Fig. 32) shows the 

 tonsillar fold (Ton.F.) more nearly erect than in the cat. In outline 

 it is approximately quadrangular and its outer (= ventral) surface is 

 less concave than in the cat. Ventrally it is limited by the alveolo- 

 lingual ridge (A-L.F.), which at this stage no longer forms the lower 

 line of the pharynx, but lies on the outer side of the giosso-epiglottic 

 fold (vallecula glosso-epiglottica). 



In the 31 mm. cat (jSTo. 500, Harvard series. Figs. 68-69) the ton- 

 sillar fold (Ton.F.) has essentially the same form as in the 23 mm. cat. 

 As in the pig last described, its lower boundary- — the alveolo-lingual 

 ridge — now lies on the outer side of the giosso-epiglottic fold (G.Ep.). 



The palatal constriction has now completed the division of the 

 pharynx into nasal and oral portions. 



The narrow cord shown in the figure parallel with the tonsillar fold 

 is an epithelial structure which lies free in the connective tissue to the 

 outer side of the fold. Its significance I have not been able to solve. 



In none of the stages so far studied did I observe any clear indica- 

 tions of the formation of lymphoidal tissue in connection with the 

 tonsillar fold. 



In the rabbit of 21 days (Fig. 73) the tonsillar fold (Ton.F.) is 

 approximately vertical. Its lateral surface is deeply concave and lies 

 between a dorsal and a ventral fold. The former corresponds to the 

 supra-tonsillar recess and evidently represents the derivative of the 

 second pouch. The ventral fold I am inclined to homologize with the 

 infra-tonsillar recess (Y.T.) which is a derivative of the pharynx. 

 Hammar, however, who describes a similar stage in the rabbit as well 

 as in man and several other mammals, fails to mention this fold as the 

 part in question. I regret that with the relatively few later stages at 

 my disposal I have not been able to solve this problem satisfactorily. 



