198 



DEVELOPMENT OF THE LIVER. 



OF THE LIVER. 



The first appearance of a bile-secreting organ is the occurrence of yel- 

 Rudiment of l w CQ ^ S variously scattered upon the lining membrane of the 

 the liver. digestive cavity, as in the hydra. A concentration or local- 

 ization next ensues, such yellow cells being grouped upon the wall of the 

 intestin3 at a definite spot. A ccecal projection, in the higher tribes, 

 seems next to force out the yellow cells, bearing them on its exterior, as 

 in the nudibranchiate gasteropods ; and as these cceca are prolonged 

 more and more, so, in a more definite manner, does the rudimentary liver 

 appear. In molluscs this partition is sufficiently distinct. The special 

 form which the hepatic apparatus presents in different tribes varies very 

 greatly, though doubtless the principle of construction and of action is 

 always the same. Thus, in insects, the liver con- 

 sists of long tubes of delicate membrane, covered 

 with secreting cells, small and germ-like near the 

 distant end of the tube, but more perfect at the 

 mouth. These tubes are in relation with an adi- 

 pose mass, which is probably connected with the 

 origin of the cells. The different condition of 

 these cells, when compared at the bottom and at 

 the mouth of the bile-sac, is well seen in the 

 case of crustaceans, as in Fig. 82, one of the he- 

 patic coeca of the cray-fish. The letters at the 

 side show the state of the cells in different posi- 

 tions toward the mouth of the follicle. At a they 

 contain yellow biliary matter only ; at b, oil glob- 

 ules are appearing in them, which become more 

 distinct at c / and toward d and e they present 

 the appearance of ordinary fat-cells. Thus, ex- 

 amined at the bottom of the follicle, the cells are 

 Biliary, an d as we Advance to the mouth they be- 



^' 



i of cray-fish. 



Hepatic 



come fatty. (Leiby.} 



The comparative anatomy of the liver is repeated in its order of devel- 

 Development opment in the high vertebrated animals. In them it is first 

 of the liver, detected in an evolution of cells upon the intestinal wall, at 

 the point which is eventually to be the place of discharge of the common 

 bile-duct. This agglomeration of bile-cells is next seen to project or bud 

 off through the intrusion of a coecal pouch. In the amphioxus the con- 

 dition thus reached remains permanent, and is the counterpart of the liver 

 of a fowl about the fourth day of incubation. The coecal pouch next 

 sends forth ramifications, which are likewise accommodated with cells, 

 and these, branching again, give origin to a complicated structure. In 



