DEVELOPMENT OF PETBOMYZON FLDVIATILTS. 199 



the stomacli, mid-gut, and hind-gut, but as the most anterior 

 of these is the narrowest part of the whole intestine, it would 

 perhaps be better to call it oesophagus. This part of the ali- 

 mentary canal lies entirely in front of the yolk, and is, with 

 the anterior region which subsequently bears the gills, raised 

 from the rest of the egg when the head is folded oflF. In my 

 later larvae it is composed of a single layer of very high co- 

 lumnar cells, and is ciliated throughout. Round this is a thin 

 layer of cells, which, I imagine, give rise to the muscular 

 coats. The whole is supported by a dorsal mesentery, each 

 side of which lies the head kidney (fig. 25). The ciliated 

 columnar cells are directly continuous with those covering the 

 dorsal ridge of the branchial region, but not with those of the 

 ventral groove ; this later connection must arise subsequently, 

 as Anton Schneider describes it in the fully-grown 

 Ammocoete. 



The mid-gut which follows the oesophagus is, in the 

 younger stages, crowded with yolk granules. The cells of tlie 

 roof soon acquire a columnar shape, whilst the ventral part 

 consists of a mass of cubical cells, each crowded with yolk. 

 By degrees the yolk is absorbed, and the cells assume the same 

 character as those lining the oesophagus. The lumen of the 

 mid-gut is very much larger than that of the oesophagus, the 

 alimentary canal expanding suddenly at the commencement 

 of the former. The absorption of yolk takes place from before 

 backward, so that lumen and walls of the fore part of the mid- 

 gut assume their permanent size and form, whilst the posterior 

 half is choked with yolk. The lining high columnar cells are 

 ciliated and quite continous with those of the ossophagus. 



By the time the yolk is all absorbed a longitudinal invagi- 

 nation of the wall of the mid-gut takes place. This occurs 

 anteriorly on the left side, but twisting through a quarter of 

 circle it comes to lie in the ventral side posteriorly. The 

 ridge thus formed reduces the lumen of the alimentary canal 

 from a round to a reniform shape in section. In this ridge or 

 spiral valve runs the subintestinal vein, which has become 

 quite small and has lost its median ventral position. Around 



