INTESTINAL CANAL. 175 



place at the point where the entoderm-vesicle and the rudiment of 

 the hind-gut are in contact, and where soon an entodermal epithelial 

 plate can be seen (Fig. 63 B, ep, p. 131). The same has been 

 •observed at the point from which the formation of the hepatic tubes 

 commences. Separate centres of epithelial formation are to be found 

 at the anterior, lateral and posterior parts of the mid-gut correspond- 

 ing to the anterior, lateral, and posterior hepatic lobes of the adult 

 animal; at these points, the epithelium soon grows out to form the 

 primary hepatic tubes. The rudiment of the posterior pair of tubes 

 seems from the first to be connected with the entoderm-plate described 

 above. As the epithelium continues to grow over the rest of the 

 entoderm-vesicle, the central part of the alimentary canal arises, 

 receiving the efferent ducts of the mid-gut gland (liver) ; this central 

 part, in the adult, is of no great extent. Its musculature arises by 

 the deposition of mesodermal elements. 



The stomodaeal invagination soon becomes divided into a narrower 

 oesophageal portion and an inner and more swollen portion, the 

 rudiment of the so-called stomach. In the latter can be recognised 

 the rudiments of the tooth-plates as epithelial thickenings, and those 

 of the gastrolith-sac as two diverticula diverging on the ventral side. 

 The young Astacus hatches with two completely developed gastroliths 

 (Reichenbach). The mid-gut becomes connected with the fore- and 

 hind-guts only at a late stage. 



The mid -gut develops in a similar manner in those Decapoda in which the 

 entoderm-sac does not retain its continuity, but breaks up into single cell- 

 elements, which become distributed in the food-yolk (Palaemon, Eupaguriis, 

 Eripliia, Atyephyra, Crangon, etc.). In these also, the entodermal elements 

 finally rise to the surface, and yield the mid-gut in the way described above. 

 Here also the first appearance of this epithelium was observed in contact with 

 the blind inner end of the proctodaeal invagination (Fig. 64 C, ep). Three 

 pairs of originally distinct hepatic rudiments, however, appear to be added 

 to it {Crangon, Kingsley). 



The formation of the mid-gut in the Arthrostraca differs from 

 the type above described as occurring in the Decapoda in that here 

 the mid-gut epithelium does not proceed from elements scattered 

 in the food-yolk, but from a paired lateral mass of cells which lies 

 superficially on the yolk and gradually grows round it (p. 139, etc.), 

 while within the yolk, vitellophags are found only in isolated 

 cases (Oniscus, Nusbaum) ; in other cases -(Porcellio, Amphipoda), 

 cell-elements are here altogether wanting. By the gradual circum- 

 crescence of the yolk from the two sides by the paired entoderm- 

 rudiment, the mid-gut vesicle closes, the very large primary hepatic 



