The Pharyngeal Pouches in the Mammalia 229 



tion, this is a more appropriate name for it than the term vesicula 

 thymica, applied by Kastschenko. 



In a 13 mm. pig, in consequence of the passive behavior of the sinus 

 and the continued outgrowth of the surrounding parts, the sinus is 

 still deeper and its margins are so near each other that, with the excep- 

 tion of its most external part, the entire sinus may be considered as 

 included in the fundus (Pig. 13). The external opening of the latter 

 is now relatively small. On its dorsal side the adjacent borders of the 

 third pharyngeal arch and anterior cervical region have fused, leaving 

 only a faint groove to mark the earlier extension of the sinus in that 

 direction. On its ventral side the part of the head underlying the 

 sinus has grown out and has united with the ventral extremity of the 

 third arch, at the same time obliterating the groove earlier connecting 

 the sinus with the pericardio-cervical fissure. 



The outer limit of the fundus is approximately formed by the middle 

 of the third pharyngeal arch. Prom this point it extends inwards and 

 slightly backwards as a deep pocket, the blind inner extremity of which 

 — the vesicula prsecervicalis — terminates close to the inferior ganglion 

 of the vagus. This part is formed by the third and fourth pharyngeal 

 grooves and the intermediate fourth arch. In consequence of the 

 diminished depth of the fourth groove, the fourth arch does not form 

 as prominent a convexity as in the preceding stage. 



In the 14 mm. pig (Pigs. 14-16 and 18) all that remains of the 

 sinus prfficervicalis has become, by virture of its passive deepening and 

 constriction, included in the fundus prsecervicalis (F.Pc), which now 

 forms a deep, blind pocket opening to the exterior by a much reduced 

 opening placed immediately under the posterior rim of the hyoid arch. 

 The outer half of the fundus forms a relatively narrow duct — the ductus 

 prsecervicalis of Kastschenko — leading to the external opening (D.Pc). 

 The inner half is relatively wider and at its mesial end is continued 

 into the vesicula (V.Pc). In this part the earlier prominence of the 

 fourth arch has flattened out and thus the third and fourth grooves 

 cease to be longer distinguishable. In this way the inner end of the 

 fundus assumes the form of a bulb. 



In the next stage, i.e., a 17 mm. pig, the narrow ductus forms a solid 

 cord, which has just severed its connection with the external ectoderm 

 (Figs. 20-21). The inner portion of the fundus now forms a relatively 

 broad, flattened band, which is slightly concave on its posterior side 

 (Pig. 21, P.Pc). By its anterior wall it is closely connected with the 



