508 DR MASTERMAN ON THE FURTHER ANATOMY AND 



recognised as the two pharyngeal clefts (p.c). In fig. 3 the first pair of oral grooves 

 (3) have bent round so as to lie completely within the pharynx, and in fig. 4 the other 

 two (2, 1) show similar relations. Between the two pharyngeal pouches, in the 

 mid-ventral line, a slight depression can be seen in this figure, and it can be followed 

 into the ventral alimentary portion of the pharynx, which is a conspicuous feature in 

 the remaining figures. In figs. 5 and 6 the oral grooves may be seen to become 

 merged dorsally into a large dorsal hyper-pharyngeal groove, which is at first partially 

 divided into two by a median ridge. Further back (fig. 7) this groove extends across 

 the middle line, and forms the median dorsal portion of the pharynx, which receives the 

 duct of the sub-neural gland, sn.gl. (cf. plate 24, fig. 13, loc cit.). This gland is seen 

 in figs. 5 and 6. In fig. 7 it has merged into the large dorsal chamber. 



In this figure the pharynx presents a symmetrical appearance. Dorsally lies the 

 large dorsal chamber, into which opens, from in front backwards, the duct of the 

 sub-neural gland, sn.gl. (in the median line), and (laterally) the two grooves which are 

 formed by the union of the three oral grooves on each side. 



The two pharyngeal pouches {p.c) lie laterally and pass towards the ventral surface, 

 whilst mid-ventrally is the large alimentary portion of the pharynx. 



A little dorsal to each pharyngeal pouch has appeared a small groove with chordoid 

 walls, which rapidly increases in size (figs. 8 and 9, n.c), and proves to be the 

 commencing pleurochord. In fig. 9 this pleurochord is seen to occupy the dorso-lateral 

 part of the pharynx, the pharyngeal pouch being ventro-lateral. After fig. 9 the latter 

 rapidly decreases in size, as seen in sections, and the pharynx then presents the appear- 

 ance indicated in plate 25, fig. 17, loc cit. 



Other important points may be noticed in a series of transverse sections, such as 

 those shown in figs. 90-99. 



Fig. 100 is a semi-diagrammatic reconstruction of the pharynx of Cephalodiscus, 

 viewed from the left side. The lines cutting it transversely indicate by their numbers 

 the approximate level at which each of the sections in figs. 90-99 are cut. 



Fig. 90 cuts the head (or pre-oral part) of the pharynx, into which falls the 

 sub-neural gland (sn.gl.). The lumen of the latter is directly continuous with the four 

 grooves here shown, two dorsal grooves (d.g.), and two ventral (v.g.). 



Figs. 91 and 92 show the two pairs of grooves to become more pronounced. A 

 condition very similar to these sections, especially to fig. 92, is shown in plate 24, 

 fig. 12, of my former paper, though the significance of these grooves had not then been 

 followed out. 



Fig. 93 cuts through the mouth-opening and through the front part of the main 

 bulk of the pharynx. Between the dorsal and the ventral grooves, the pleurochordal 

 diverticulum (n.c.) and the commencing pharyngeal cleft (p.c.) have now become 

 conspicuous features on either side of the section, so that the grooves are widelyi 

 separated. The dorsal grooves unite more or less in one, and the ventral grooves nil 

 down laterally on either side of the mouth (v.g.). They were distinctly figured in thii 



