HISTOGENESIS OF THE LIVER 



261 



In the rapidly growing parts of the hver the cyHnders often 

 terminate peripherally in expanded end-bulbs or vesicles which 

 contain a lumen many times the diameter of that of the typical 

 cylinder. Such vesicles are found in embryos from 25 to 40 mm. 

 in length but not in older specimens. Their walls consist of a 

 low columnar or cuboidal epithelium. Figure 4 is a cross section 

 of one of these structures from the lateral lobe of an embryo 

 36.6 mm. long (S.C. 10). The significance of these vesicles is 

 unknown to me. 



V L X L ^ 



Fig. 4 Terminal vesicle of a hepatic cylinder from the lateral lobe of an Acan- 

 thias embryo 36.6 mm. long (S.C. 10). X 400. L, lumen of hepatic cylinder; 

 V, terminal vesicle; X, side branch from central lumen of cylinder. 



6. Changes in the size and structure of the hepatic cylinders. 

 When the hepatic tubules enter upon anastomoses they are 

 irregular in form and of variable caliber. The lumina of the 

 tubules are large and irregular and are generally surrounded in 

 cross section by 14 or 15 cells, if one may judge from the number 

 of nuclei present, for cell boundaries are often indistinct at this 

 time. 



Table 1 shows some of the changes which take place in the 

 course of the later development of the cylinders. The measure- 

 ments and cell counts given in this table are in each case averages 

 determined from twenty fair cross sections of cylinders of the 



THE AMBRICAN JOURNAL OF ANATOMY, VOL. 17, NO. 3 



