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399 



pouches. The 1st pair of visceral pouches are shown as prominent 

 ridges running outward and backward. In the model of the cavity 

 only, they project much farther free than in the one in which the model 

 includes the mucous membrane. A para-sagittal section of this region 

 of the embryo would show a relatively large opening, then a narrow 

 one running dorsally and connecting with the cavity of the 2nd visceral 

 pouch. This cavity (shown as a ridge on the model), representing the 

 oral part of the 1st visceral pouch was called by Moldenhaur the sulcus 

 tubo-tympanicus. Hammar uses the same term to mean the oral 

 lengthening and development of the 

 1st visceral pouches as first used by 

 Moldenhaur. Compare this structure 

 in Figs. 2, 3, 4 and 5. Seen directly 

 from behind (dorsally) all four pairs 

 of visceral pouches are visible. The 

 1st pair running slightly upwards 

 (cephalic). The 2nd pair show as 

 two ridges flattened dorsoventraily 

 and projecting downward. The 3rd 

 pair stand out at right angles to the 

 oesophagus and the cells have already 

 begun to proliferate around it as a 

 comparison of the two models will 

 show. In the one in which the endo- 

 derm is included this pair of pouches 

 is very much larger and exhibits pro- 

 cesses v/hich are not suggested by the 

 model, showing only the cavity. The 

 4th pair are peculiarly shajX'd struc- 

 tures projecting at right angles to 

 the oesophagus and then bending 

 sharply dorsally on themselves. Their 

 ends are enlarged and their outline is 

 nearly triangular. They are rela- 

 tively much larger in tlie model that includes the mucous membrane 

 than in the one of the cavity only. In this model, however, they 

 do not show a tendency to divide into two prominences as in embryo II 

 or in others to be described later. Just beneath the 4th visceral pouch 

 the whole pharynx constricts notably and marks off sharply the begin- 

 ning of the oesophagus. See Fig. 4. A section of the pharynx taken 

 at any point between the dorsal angle and the prominence of the 4th 



Fig. 6. View of reconstruction of 

 embryo CLXIII from a ventral-caudal 

 view. B. v., blood vessel; Cb., note- 

 chord; L., rudiment of larynx; Max., 

 superior maxillary process; N., nasal 

 fossa; N. X., vagus nerve; Ph., pharynx; 

 S. p., sinus praecervicalis (His) ; V. F/, 

 V. F.^^, V. Y.'"^ first, second and third 

 visceral folds. Section on a line indi- 

 cated by A. B., Fig. 4. 



