THE ORGANS OP THE INNEK GERM-LAYER. 



327 



duct is added in the course of another day. Also in the case of 

 human embryos 4 mm. long His demonstrated that at first there is 

 only a single hepatic duct, and that some time afterwards a second 

 appears (fig. 163 Lbg). 



In the further course of development both the unpaired and the 

 paired hepatic fundaments are metamorphosed quite rapidly into 

 a tubular gland with numerous branches ; this acquires a special 

 character, differing from that of simple tubular glands, owing to 

 the fact that the tubes early become joined together to form a fine 

 network, since the primitive hepatic tubes send out numerous 

 lateral buds, which in some Vertebrates 

 (Amphibia, Selachii) are from the be- 

 ginning hollow, in others (Birds, Mam- 

 mals, Man) solid. Imbedded in the 

 embryonic connective substance of the 

 ventral mesogastrium, they grow out in 

 the former case into hollow tubes, in 

 the latter into solid cylinders. These 

 in turn are soon covered with corre- 

 sponding lateral processes, and so on. 

 Inasmuch as these grow toward one 

 another, and where they meet (fig. 187 k) 

 fuse, there arises a close network of 

 hollow glandular canals or solid hepatic 

 cylinders in the common connective- 

 tissue matrix. 



Simultaneously with the epithelial 

 network there is formed in its meshes 

 a network of blood-vessels (g). From 

 the vena omphalomesenterica, which, 



as previously stated, is embraced by the two hepatic tubes, there 

 grow out numerous shoots, and these by forming lateral branches 

 unite with one another in a manner corresponding to that of the 

 hepatic cylinders. 



The liver of the Chick is found to be in this condition on the sixth 

 day. It has become even now a rather voluminous organ, and is 

 composed, as in the case of Mammals and Man, of two equally 

 large lobes, each of which has arisen from one of the two primitive 

 hepatic ducts by budding. The two lobes produce on the ventral 

 mesentery two ridges, one of which projects into the left body-cavity 

 and one into the right (fig. 184). 



Tig. 186.-Diagrammatic view of the 

 alimentary canal of a Chick on 

 the fourth day, after GOETTE. 



The heavy line indicates the inner 

 germ-layer, the shaded portion 

 surrounding it the splanchnic 

 portion of the mesoblast. lg, 

 Lung ; St, stomach ; p, pancreas ; 

 I, liver. 



