HARRIMANIA MACULOSA II9 



rapidly and incline downward, until at the point of communi- 

 cation with the esophagus the isthmus is a broad chamber, much 

 depressed and applied ventrally close against the dorsal esoph- 

 ageal wall (fig. 6). At this point a prominent ridge begins 

 in the middle of the dorsal wall of the isthmus (w. r.). At 

 its anterior end this ridge is sharply triangular in section with 

 its apex pointing downward into the narrow slit between the 

 closely approximated skeletal crura (fig. 7, in. r.). 



This is the point at which the nuchal notochord passes di- 

 rectly over into the esophageal notochord. Fig. 7 presents a 

 characteristic appearance of a section of the esophageal noto- 

 chord a short distance behind its beginning. By the ridge 

 [m. r.) mentioned above, it is divided into two well-marked lat- 

 eral, pocket-like portions (/. ^, and r. p.). The ridge is com- 

 posed of much elongated, closely appressed epithelial cells with 

 their nuclei closely crowded in the apical part of the ridge. 

 This ridge broadens and becomes relatively lower as it extends 

 backward (fig. 4). The walls of the lateral pockets are 

 composed of much vacuolated cells, with comparatively few 

 nuclei confined to their inner ends. The histological similarity 

 between the walls of these pockets and the nuchal notochord is 

 perfect. In fact, as the description shows, the continuity from 

 the one to the other is nowhere broken. The skeletal crura 

 {sk.c.) mark the region at which the esophageal wall passes 

 over into the notochordal pockets, as the figures show^, and as 

 these pass backw^ard they diverge from each other and trend 

 gradually ventrad (figs. 6, 7 and 4). 



From this disposition of the crura the esophageal notochord be- 

 comes gradually broader, this broadening being effected chiefly 

 by the broadening of the median dorsal ridge already described — 

 the pockets retaining nearly their same size and character far 

 back. As has been stated in the diagnosis, the skeletal crura reach 

 into the pharyngeal region and overlap two or three branchial 

 bars (fig. 3). As their posterior ends lie some distance ventral 

 to the branchial apparatus, it follows that the first two or three 

 branchial orifices are in the portion of the esophageal wall that 

 is a direct backw^ard continuation of the esophageal notochord. 

 In fact, one might say that the extreme posterior portion of the 



