HISTOGENESIS OF THE LIVER 253 



to a single row which Hes near the external surface of the epithe- 

 lium. There seems to be but a single layer of very high columnar 

 cells at this point but the cell boundaries are not very clear. 

 There is always a broad clear zone of cytoplasm towards the 

 lumen of the pouch opposite the tubule ridge. Mitotic figures 

 do not occur in the ridges but are frequent in the zones of epithe- 

 lium between them. Following the formation of the ridge there 

 appears a very shallow trench on the internal surface of the 

 epithelium. The lumina of the individual tubules appear as the 

 tubules themselves are differentiated through the breaking up 

 of the ridges into rounded anlagen. One then finds in each tubule 

 anlage a narrow slit-like cleft passing between the cell walls 

 at right angles to the pouch cavity. This is shown in figure 29. 

 Thus at first the tubule cells do not lie at right angles to the tubule 

 lumen but nearly parallel to it. Later the cells assume an oblique 

 position (see again fig. 29) and finally come to lie in the typical 

 radiating position in relation to the lumen (fig. 30). As the 

 tubules develop the cells become shorter and their nuclei change 

 from elongately ovoid to nearly spherical bodies. 



3. The anastomosis of the hepatic tubules 



The liver is transformed from a gland of the branching to one 

 of the reticular type by the anastomosis of its end pieces. The 

 process begins in Acanthias in embryos from 12 to 15 mm. in 

 length. Such embryos have from 70 to 85 pairs of somites and 

 correspond roughly with Balfour's stages L and K and with Nos. 

 25-26 of the Normal plate series. The tubules arising from the 

 dorsal portions of the lateral liver pouches precede in their 

 differentiation those of other regions and it is generally among 

 these dorsally placed tubules that anastomoses are first found. 

 Later the tubules of the lower parts of the lateral pouches and 

 finally those of the pars hepatica medialis enter upon the process. 

 Variants from this general plan of procedure are not uncommon. 

 Figure 2, of a sagittal section of the left hepatic pouch of an 

 embryo 14 mm. long (S.C. 30) shows how general anastomoses 

 are when once they begin in a given region. 



