306 PROFESSOR FRANK J. COLE 



There can be no doubt that the folds of this portion of the gut are directly continuous 

 with those of the abdominal intestine (and doubtless, therefore, have developed from the 

 same embryonic layer), which have never entirely disappeared in the flat region behind 

 the bile duct. This section, however, is only continued as far anteriorly as the opening 

 of the ductus oesophago-cutaneus, from which point forwards the character of the 

 mucosa quite changes, and, judging from anatomical evidence only, might have developed 

 from a different embryonic layer. Two at least of the folds are continued forwards 

 directly on to the posterior lining of the ductus oesophago-cutaneus, where they are pro- 

 longed outwards to its external opening. The other folds of the ductus are some of 

 them intrinsic, and some are continuous with folds belonging to the next anterior 

 region of the gut. The ductus does not open directly into the gut, but rather into a 

 small pocket-like evagination of the latter, which itself then opens into the gut at 

 right angles to the ductus. 



In front of the ductus oesophago-cutaneus is the branchial gut of Maas. Its 

 posterior boundary is the ductus, and its lining epithelium is quite "epidermal" in 

 character, which is somewhat surprising, seeing that it cannot be stomodseal in origin, 

 even if it represents morphologically a greatly elongated pharynx. Its mucosa exhibits 

 more numerous, closely-set, and shallow^er folds, some of which are continuous, or fuse, 

 with those of the afferent gill ducts. A disturbing factor here appears to be the 

 apertures of the latter ducts, and the mucosa of the branchial gut becomes increasingly 

 irregular from before backwards. This, however, cannot be entirely due to the exit 

 of the gill ducts, since between the first two the mucosa presents almost tlie same 

 appearance as it does in front. But behind this point the pattern is complicated by 

 the folds here and there gradually approximating, joining, or being connected up by 

 numerous short, shallow, transverse ridges. The result is that there is a suggestion 

 of a honeycomb mucosa at this region of the gut. Here also the wall of the gut is very 

 thin, and the mucosa not detachable as in the abdominal intestine. 



In front of the gill region, which must, from its development, be regarded as 

 secondary hrancliial gut, the mucosa presents a very regular appearance, and there are 

 about fifteen moderately prominent and perfectly straight longitudinal folds. These 

 folds, however, do not always pass continuously from one end of this section of the gut 

 to the other, but here and there die down, and are replaced by others. Opposite the 

 posterior free extremity of the pharyngeal velum the folds tend to anastomose so as to 

 form a simple honeycomb pattern, and they here terminate — none of them being 

 prolonged into the mouth. 



The velum, or pharyngeal valve, according to Huxley, marks off the posterior 

 boundary of the mouth. It may be described as an extensive fiat dorsal duplication of the 

 mucosa, situated just behind the dental apparatus, when the latter has been withdrawn, 

 and attached to the mucosa of the roof of the mouth by its mid-dorsal surface i)i much 

 the same way as the gut is suspended by the mesentery. Now, this suspensory fold 

 is prolonged backwards behind the region of the velum, and passes insensibly without 



