246 PROCEEDINGS OF THE ACADEMY OF [Marcb, 



the pharyngeal wall and 'the tubal Anlage. Immediately beyond 

 this region this conuecting'portiun broadens out considerably to form 

 a rather shallow bulging^ of cells from the roof of a "lateral 

 recess" (fig. 25, l.r.) or pouch of the pharynx, from the outer 

 extremity of which the thymus gland is given off. This " lateral 

 recess " is really formed by an extension outwax-d of the branchial 

 portion of the pharynx over the internal branchial openings, so- 

 that the latter are now situated on the floor of the cavity. In the 

 preceding stage this " lateral recess" was just beginning to form 

 as a slight bulging beneath the proximal portion of the tubal 

 Anlage. With the extension externally of the " lateral recess" 

 the proximal portion of the Eustachian cord comes to appear as 

 an inconspicuous protuberance over the inner part of the roof of 

 the " recess" (fig. 25, immediately internal to vii h). 



Stage VI. — Tadpole of 18 mm. Posterior limbs appearing as 

 minute buds below the root of the tail (PI. IX, fig. 26). 



In this stage the Eustachian cord has about reached the height 

 of its degeneration. The cord still maintains the same general 

 relations to the surrounding parts as before. Its distal, expanded 

 extremity remains distinct, and from thence the cord can readily 

 be traced to its characteristic position next to the outer surface of 

 the processus muscularis. Here, however, it soon becomes very 

 small and then can be traced only with the greatest difficulty. 

 The cord lies immediately above the mandibular aorta, and by 

 following the latter it may be traced as a minute, more or less 

 flattened pigmented patch, which in certain parts contains one or 

 two nuclei not readily distinguishable from the nuclei of the sur- 

 rounding fibrous tissue (fig. 26, En.). Immediately posterior to 

 the quadrato-hyoid articulation the cord again enlarges slightly 

 and can be traced thence for a considerable distance. Then in the 

 region where the ramus hyomaudibularis begins to come into close 

 relations with its external surface all distinct traces of the cord are 

 lost. Nothing more of the cord is to be made out until we come 

 to the region where the mandibular artery turns inward, where 

 for a short space the cord is again revealed and then terminates 

 without forming any clear connection with the pharyngeal wall. 

 This most posterior fragment of the cord is situated below the 

 quadrate, dorsal to the upper anterior end of the " lateral recess " 

 of the pharynx. There is no distinct proliferation from the dorso- 



