1902.] NATURAL SCIEXCKS OF PHILADELPHIA. 93 



Fig. 5e sliows the wide separation of the dorsal and ventral 

 branches of the ciliated grooves, so that the two dorsal branches are 

 now quite near together at the mid-dorsal line (d.c.g.), while the 

 two ventral branches also are seen to approach each other as they 

 pass posteriorly along the walls of the pharynx (v.e.^r. ). The 

 dorsal grooves continue to approach each other gradually as they 

 are followed posteriorly until they are separated only by a median 

 ridge of ciliated columnar cells (fig. od). At a point a short dis- 

 tance anterior to the thyroid, the grooves disappear and the medial 

 ridge continues posteriorly, though diminished in height, as the 

 cilixited dorsal ridge (fig. 5^, c.d.r.). This ridge continues to the 

 extreme posterior end of the pharynx and enters the oesophagus, of 

 which it forms, for some distance, the ciliated dorsal wall. 



Tlie ventral grooves, as has been said, approach the mid-ventral 

 line of the pharyngeal wall until they unite and lead into the duct 

 of the gland (figs. 5e-i). Posterior to the opening into the thyroid 

 they are continued on the floor of the pharynx as a single, shallow, 

 median groove (figs, ^j—k), which finally disappears, posterior to 

 the thyi'oid, as a groove, but continues to the oesophagus as a ridge 

 of thickened epithelium. In an embryo cut sagitally, so that the 

 phaiynx may be laid open and exposed from above, the ciliated 

 grooves may be seen easily Avith the naked eye, and still more 

 plainly with a lens, but no indication of grooves or bands running 

 from the dorsal ciliated ridge to the ventral groove is to be made 

 out. A transverse section through one of the gill-arches showed 

 the presence of ver}'' fine short cilia, but their arrangement could 

 not be made out. This arrangement of ciliated grooves h quite 

 different from that described by Willey in his interesting book, 

 Amphioxus and the Ancestry of the Vertebrates. On p. 168 he 

 says : ' ' These grooves curve forward and upward in front of the 

 gill-clefts (after the obliteration of the first gill-pouches), and then 

 proceed backward on either side of the dorsal middle line of the 

 pliarynx as far as the commencement of the oesophagus. Here 

 they appear to cun'e downward again, and, uniting togetlier, extend 

 forward as a median ventral groove to the posterior lip of the hypo- 

 branchial aperture. ' ' 



The arrangement of the ciliated bands and grooves, then, is 

 briefly as follows: on the floor of the pharynx, beginning at or 

 near the opening of the oesophagus, is a ridge of epithelium on 



