111 LIVER 187 



greatly elongated and branched, and gives the gland a tubular 

 character. This character may be retained throughout life (Lampreys) 

 but normally the tubules undergo anastomosis so as to form a net- 

 work of trabeculae. While this is to be regarded as the primitive 

 mode of development of the tubules it is to be noted that they more 

 usually in actual fact show the modification of development which 

 we have learned to associate with the presence of yolk, being at first 

 solid and taking their origin not by a process of outgrowth but rather 

 by a process of modelling by ingrowing mesenchyine. 



In the meroblastic Vertebrates also the liver may be described 

 as originating from a mid-ventral outpushing of the enteric wall. 

 Variations occur in detail, in correlation with the varying relations 

 of the hepatic portion of enteric wall to the fore-gut and yolk-sac. 

 If this part of the gut-wall has already been folded off from the 

 yolk-sac and incorporated in the fore-gut, then the early stages of 

 development of the liver diver ticulum pursue their normal course. 

 If, on the other hand, it still forms part of the yolk-sac wall, the 

 hepatic rudiment makes its appearance as a projection from this, and 

 it may be in its first beginnings paired, its two halves separated by 

 the longitudinal slit by which the cavities of the definitive gut and 

 the yolk-sac are still continuous. 



ELASMOBKANCHII. The hepatic diverticulum at an early stage 

 bulges out to form a conspicuous outgrowth on each side anteriorly 

 the rudiments of the right and left lobes of the liver. The median 

 portion between these becomes in its anterior region converted into 

 secretory tissue while its posterior part becomes the bile-duct, with 

 its dilatation the gall-bladder. 



In Acanthias (Scammon, 1913) the first rudiment of the liver, 

 which makes its appearance at a time when this region of the 

 enter on is not yet floored in but opens freely into the subjacent 

 yolk-sac, is distinctly paired. In view of the unpaired condition in 

 Amphioxus and the holoblastic Craniates there can be little doubt 

 that this condition in Acanthias is a secondary modification as 

 indicated above. Secondary pockets soon make their appearance on 

 the wall of the secretory portion of the rudiment, and grow actively 

 into elongated and much -branched tubules. These fuse together 

 secondarily to form the network characteristic of the fully developed 

 liver. This network is bathed by the blood of the vitelline veins 

 (see Chap. VI.). 



After the embryo (Acanthias) has reached a length of 25-28 mm. 

 the walls of the tubules, or trabeculae of the network, increase greatly 

 in thickness so that both their own cavities and the intervening 

 blood-spaces become relatively reduced and the organ assumes the 

 compact definitive condition. 



Whereas the tubules become throughout the greater part of their 

 extent secretory in function the proximal portions, each common to 

 a group of tubular branches, function merely as ducts. These communi- 

 cate with the main bile-duct formed from the posterior and median 



