1901.] NATURAL SCIENCES OF PHILADELPHIA, 245 



that in the precediug stage. The tissues of the animal are more 

 compact and definitely limited than iu the last stage. The 

 external, distal extremity of the " diverticulum" (or, as I may 

 now term it, the Eustachian cord, since the structure under con- 

 sideration ultimately gives rise to the greater part of the tube of 

 that name) has the same general appearance as before. It, how- 

 ever, does not extend so far out from the processus muscularis as 

 before, a condition probably produced by the increase in size of 

 the process. A slight reduction has also taken part in this portion 

 of the cord (" diverticulum "). More marked, however, has been 

 the change in the middle portion of the cord. After extending 

 inward to the processus muscularis the cord rapidly degenerates, 

 becoming greatly flattened and much reduced in size, so that for a 

 part of its course it is very difficult to recognize, the presence of 

 scattered nuclei and numerous pigment granules alone serving to 

 mark its existence. This great reduction has been associated with 

 a continuation of the processes described in the last stage, i.e., the 

 growth of the hyoidean muscles (depressor maudibuloe and depres- 

 sor ossis hyoidei), the articulation of the hyoid with the quadrate 

 and the inci'ease in size of the mandibular artery (PI, VII, fig, 8 

 and PI, IX, fig, 25, En.). 



The cord retains the degenerate condition just described until it 

 reaches a point just back of the region where the nmudibular aorta 

 turns inward to join the carotid. A good idea of the condition of 

 the cord can be obtained from coronal sections (fig. 8). In such 

 it appears as a faint, narrow cord (£"«.), coursing in an antero- 

 posterior direction in contact with the outer surface of the pro- 

 cessus muscularis. This cord contains no lumen and shows no 

 indication of a tubal character. It contains throughout its course 

 scattered nuclei arranged end to end, and it is largely colored by 

 numerous black pigment-granules. The yolk-spherules have now 

 disappeared entirely. There is very little substance to the cord 

 and in places where nuclei and pigment are lacking it becomes 

 very ditScult to trace. 



Immediately posterior to the inflexed mandibular aorta the 

 Eustachian cord is joined to the pharyngeal wall by a narrow 

 strand of somewhat elongated cells. These cells are not easily 

 distinguishable from the cells of the surrounding connective tissue, 

 but they form a i-ather dense patch in the latter stretched between 



