APPENDAGES OF THE MID-GUT OF VERTEBPATA. 563 



the Monotremata only, among Mammals, in the rest of which it is 

 confined to the embryonic stage, and subsequently the hind-gut 

 opens to the exterior by means of an anus, 



Organs appended to the mid-gut. 

 §421. 



Two large glandular organs, the liver and pancreas, are 

 connected with the beginning of the mid-gut ; they are both dif- 

 ferentiated from the walls of the rudimentary enteron. 



In Amphioxus an organ, which must be regarded as the liver, 

 has the form of a caacal tube (Fig. 303,/), which arises close to the 

 commencement of the alimentary canal, and is directed forwards 

 (Fig. 303,/). It is provided with an epithelial investment of a 

 greenish colour. A similar condition is 

 seen in the Craniota during the earliest 

 stages of development, in which the rudi- 

 ment of the liver has the appearance of a 

 paired diverticulum (//) of the enteric 

 tube, lying behind the rudimentary stomach 

 (Fig. 319, J). It is partly formed by the 

 epithelial layer of the rudimentary enteron 

 (endoderm), and partly by the external layer 

 developed from mesoderm. As Reptiles, 

 Birds, and Mammals agree in this point, 

 this condition must be regarded as a funda- 

 mental one, while at the same time it calls 

 to mind the morphological characters of 

 the hepatic organ in Amphioxus and many 

 Iuvertebrata (Vermes, Mollusca). 



Owing to the thickening of the splanch- 

 nopleure and its large connection with the 

 venous portion of the vascular system, 

 together with the simultaneous thickening 

 of the endoderm, relations are produced, 

 which distinguish the liver of the Craniota 

 from that of the Acrania, as well as from 

 that of the Invertebrata. While the first 

 rudiment of the liver appears as a diver- 

 ticulum, the later differentiations are 

 brought about by the thickening of the 

 endoderm, and give rise to solid chords of 



cells which grow into the layer of mesoderm, and the vascular 

 apparatus embedded in it ; these give off new buds, and are finally 

 connected together in a retiform manner. The parenchyma of the 

 liver is formed by these primitively solid chords, and their secondary 



o o 



a O -i 



Fig. 319. Rudiment of the 

 enteric canal and its ap- 

 pendages in an embryo 

 of the Dog, seen from the 

 ventral surface, a Diver- 

 ticula of the enteric tube 

 towards the visceral clefts. 

 b Rudiment of the pharynx 

 and larynx, c Of the lungs. 

 d Of the stomach. / Of the 

 liver, g Walls of the yolk- 

 sac in connection -with the 

 mid-gut. h Hind-gut (after 

 Bischoff). 



