The Pharyngeal Pouches in the Mammalia 205 



dorsal apex it is interrupted by a deep incision — the incissura tensoris 

 (I.Tn.). Anterior to the apex is the submeckelian fold (SM.P.) facing 

 at this stage in the antero-dorsal direction. Laterally its margin is so 

 far upturned as to hide from view the adjacent part of the tubo- 

 tympanal border. 



The nianiibrial fossa (]\In.r.) is very dee]). It lies immediately l:»elow 

 the tensor incision^, bounded anteriorly by the submeckelian fold and 

 posteriorly by the post-manubrial fold (P-M.F.). The external audi- 

 tory tube lies a short space below the fossa, but is still separated from 

 it by a considerable thickness of connective tissue. 



The most noteworthy feature of this stage is the initial division of 

 the pharynx into its oral and nasal portions by the backward extension 

 of the palatine incisions. The oral cavity has been entirely separated 

 from the nasal tube, but the constriction of the pharynx has only begun 

 in its more anterior part. The constriction, as the figures show, takes 

 place in the part lying below the post-salpingeal fold, between it and 

 the tonsillar fold. 



The rabbit of 18 days (Harvard series), while showing a slight dif- 

 ference from the preceding, is yet so closely similar that a full descrip- 

 tion is unnecessary. Its general features can be seen by consulting 

 Fig. 73. The most marked feature is the greater posterior extension of 

 the palatal constriction. The broad grooves continued back from the 

 latter over the sides of the pharynx represent the posterior palatine 

 grooves (Arcus pharyngo-palatinus), p. pi. 



In the pig of 32 mm. (ISTo. 74, Harvard series, Pigs. 32, 33, 52) the 

 Eustachian tube is more constricted than in the preceding stage. The 

 tympanic pouch projects sharply outwards and backwards. Close to 

 where it joins the tube it gives off the still prominent submeckelian 

 fold. Where the latter and the tubo-tympanal borders meet is the 

 dorsal apex (Eecessus anterior, D.A.I). Below the latter on the 

 lateral wall is the now crescent-shaped post-manubrial fold. The Tatter 

 arches around underneath the manubrial fossa (Mn.F.) and becomes 

 continuous with the portion extending to the dorso-lateral margin of 

 the pouch. Immediately below the fossa the ventro-lateral surface of 

 the pouch is flattened and is adpressed against the inner part of the 

 external auditory tube. Only an exceedingly thin layer of connective 

 tissue intervenes between the two structures (Fig. 52). 



The cat of 31 mm. (No. 500, Harvard series. Figs. 68-69) gives the 

 final stage in its species. The Eustachian tulie is very narrow, while 



