314 PROFESSOR FRANK J. COLE 



smaller. They also occur in the hinder extremity of the naso-pharyngeal duct. At 

 the same time the glassy mucous cells increase in number, and the superficial cells 

 giving the mucin reaction so characteristic of the anterior section of the mouth 

 almost disappear, to be replaced by an epithelial mosaic. Further back, however, 

 they become more numerous again, but are never as well developed as in the mouth. 

 There is no doubt that we have here a distinct change in the character of the 

 epithelium, which corresponds to the boundary between the stomodseum and the 

 mesenteron. 



The submucosa is a very dense fibrous connective tissue immediately under the 

 epithelium, but becomes looser further away from it. The gut behind the velum 

 resembles the velar gut. 



Posterior to the velum the gut gradually narrows down to rather more than half 

 the width at the velum, and the lumen becomes correspondingly contracted and 

 flattened dorso-ventrally. It then gradually increases in calibre, the lumen becoming 

 larger and oval in transverse section. These changes, however, involve no corresponding 

 histological change. 



There is no real distinction between the pre-branchial and the branchial gut, but 

 the branchial submucosa is more vascular and not so dense, and there are rather more 

 unstriped circular muscle fibres, although they are still very sparse. Again, near the 

 hind end of the branchial gut I have observed a slight tendency for the unstriated 

 musculature to be directed into \he, folds. Both the glassy and, to a lesser extent the 

 granular, mucous cells, are still present in great abundance, but the unstriped 

 musculature embedded in the submucosa almost disappears posteriorly, and external to 

 the submucosa for a short distance we have a new layer, a wide zone of fatty tissue, 

 encircling which is a layer of unstriated muscle. Apart from the epithelium a section 

 of the gut here is not unlike one of the abdominal intestine. 



The branchial gut increases gradually in calibre from before backwards, and the 

 lumen is largest a short distance in front of the ductus oesophago-cutaneus. It then 

 narrows down markedly, increases again in the immediate neighbourhood of the ductus, 

 whilst behind the latter, and where it is surrounded by the constrictor cardise, it 

 abruptly diminishes, and is smaller and more definite in shape {i.e, circular in transverse 

 section) than in any other region of the gut. Behind the constrictor it widens again 

 gradually until the region of the bile duct is reached, and then it rapidly expands into 

 the full size of the abdominal gut. 



The transition from the branchial into the abdominal gut may be best studied in a 

 series of longitudinal sections. We then note the following points : — 



Epithelium. — No change is noticeable until about the centre of the constrictor 

 cardise, where there is a marked diminution in the granular mucous cells. The glassy 

 cells also decrease in number, but not to the same extent. Both kinds of cells, however, 

 may occur right up to the boundary between the two types of gut — i.e. between the 

 many-layered and single-layered epithelium. With the diminution of the mucous cells 



