DEVELOPMENT OF THE ELASMOBRANCH LIVER 371 



the ducts direct, all further increase in hepatic parenchyma being 

 due to growth in the hepatic trabeculae proper. 



The later histories of the major and minor hepatic ducts can 

 be most easily undertaken separately. Up to the stage just de- 

 scribed the gradual shifting of the anterior end of the ductus 

 choledochus to the right has been a constant feature. This 

 process becomes more noticeable with the distinct differentiation 

 of the hepatic ducts and by the time the embryo reaches a length 

 of from 25 to 28 mm., the anterior end of the duct may be so ro- 

 tated that the original anterior surface faces the right and the 

 left surface appears as the anterior one. The rotation though 

 always present is not always so extreme as this type which is 

 shown in figures 51 and 52, This process is brought about by a 

 distinct enlargement of the sinus formed by the omphalo-mesen- 

 teric veins which lies below and to the right of the ductus chole- 

 dochus. The rapid growth of this sinus is in turn due to changes 

 in the hepatic trabeculae. Until the embryo reaches the length 

 of from 25 to 28 mm. the trabeculae form a wide meshed network 

 and the blood is able to flow around them through sinusoids of 

 large caliber. From this stage on however the trabeculae in- 

 crease rapidly in size and the surrounding sinusoids are reduced 

 to extremely small vascular channels.^ The blood is thus directed 

 in a large part into the omphalo-mesenteric veins and their caliber 

 is considerably increased. 



The main hepatic ducts are markedly influenced by this rota- 

 tion. The left one is carried forward until almost its entire 

 length lies in the transverse plane of the embryo and only the dis- 

 tal end with the tubules arising from it project backward while 

 the right hepatic duct extends directly backward with little or no 

 lateral course (figs. 21, 51, 52 and 54). Soon after the rotation 

 of the anterior end of the ductus choledochus takes place the 

 cystic duct, which until this time has joined with the ductus chole- 

 dochus from below, is carried forward and upward along with the 

 gall bladder until it joins with the anterior surface of the ductus 

 choledochus and appears at first sight as an anterior extension of 



* This fact has already been recognized and recorded by Minot ('00). 



