HISTOGENESIS OF THE LIVER 255 



Figure 24 is of a wax reconstruction of three anastomosing 

 tubules from the left hepatic pough of an embryo 15 mm. in 

 length (H.E.C. 227). The pouch wall from which the tubules 

 spring is cut away squarely around the base of each tubule. 

 Tubules A and B are completely anastomosed end to end but 

 there is still a distinct constriction at the plane where they have 

 joined and the lumen within is considerably narrower than in 

 the bodies of the tubules. Although not clearly shown in the 

 figure both A and B are T-shaped and in each case the other 

 tip of the tubule ends freely without anastomosis. Tubule C 

 is a compound one and the subdivision which passes over to join 

 with 5 is a branch of the third order. There is as yet no actual 

 anastomosis between C and B, but the epithelial walls of the 

 two are in direct contact. 



The lack of any account of the histology of anastomosis seems 

 to warrant a rather full description of the process here. The 

 process may be rather arbitrarily divided into three steps or 

 stages. In the first step the tubules come in actual contact; 

 in the second there occurs the fusion of their walls and a re- 

 arrangement of the cells forming them, and in the third there is 

 an establishment of a connecting channel between the two 

 original lumina. 



These stages of anastomosis can be observed in the liver of 

 Acanthias embryos of any length from 13 to 45 mm. They are 

 more easily followed, however, in young specimens. The figures 

 which are used here to illustrate the process have been taken 

 from embryos under 20 mm. in length. In each case the sections 

 have been followed through and the tubules involved have been 

 reconstructed to make sure that the picture presented was not 

 due to an oblique plane of section or a misinterpretation of a 

 segment of a single complicated tubule. 



Early stages of contact of the anastomosing tubules are illus- 

 trated by figure 30 which is a transverse section of the left hepatic 

 pouch of an embryo 13.3 mm. long (S.C. 18). Numerous tubules 

 extend outward from the lateral wall of the hepatic pouch. Of 

 these the larger number are still in the form of simple tubules 

 which are expanded distally but a few have entered upon anasto- 



