94 GENEEAL SKETCH OF 



of hypoblastic cells pushed in from the side, or budded off 

 from the nucleated periblast below, probably both\ The 

 mid-gut soon, when the invagination of the lenses becomes 

 complete, forms a massive cylinder, and the oral tract a wide 

 sheet of hypoblast. By the time the ear-capsules have thinned 

 out and the otoliths have been developed, a fine fissure traverses 

 the pharj'ux, and the lumen continues to the blind end of the 

 canal. The cells become cubical and of many layers, and the 

 inner lining has a granular or mucoid appearance. The inner 

 mesoblastic sheet (splanchnopleure) e.g. gives a coating to the 

 canal — forming the muscles and connective tissue, while ex- 

 ternally it gives rise to the epithelial peritoneal layer. The 

 oesophageal tract at least appears to be covered with cilia. 



At first straight and smooth — the walls of the canal in the 

 later stages become folded and wrinkled — especially posteriorly. 

 Indeed, about 10 days or a fortnight after hatching some forms 

 show a capacious though flattened oral chamber and oesophagus, 

 the latter giving origin to the duct of the smm -bladder. The 

 enlarged stomach follo\\s, the liver lying beneath, with the 

 dense pyloric section posteriorly — the pyloric caeca subsequently 

 springing from it as evaginations, as observed in young cod 

 I to IJ inch in length. From this position also passes the 

 bile-duct. The intestinal walls are likewise dense, and rapidly 

 develop rugae and a glandular character. Posteriorly the rectal 

 region is marked by a cincture or valve, followed by an enlarge- 

 ment ; it then bends downward and narrows to form the small 

 vent — opening upon the muscvilar papilla. 



Mouth. 



An involution of the outer layer (epiblast) to form the 

 mouth does not occur in these forms. The oral cavity is 

 capacious and the branchial framework supporting its floor and 

 sides well advanced when a fissure from the chamber bursts 

 through in front. The jaw-cartilages grow rapidly forward, 

 and the mouth, at first ventral and shark-like, opens from 

 above — from the rapid forward growth of the lower jaw. 



1 Cunningham says periblast only. Q. J. Mic. Sc, vol. 26, 1885, 



