HISTOGENESIS OF THE LIVER 257 



core in which cell boundaries are faintly distinguishable. In so 

 doing the axes of the nuclei rotate through an angle of 90° so 

 that instead of being parallel to the long axis of the connecting 

 stalk as at first, they are now at right angles to it. Such rotation 

 can be determined, of course, only in young tubules where the 

 nuclei are oval and not circular in section. Figure 26, drawn 

 from an embryo 19 mm. in length (S.C. 3), shows an early step 

 in these changes. Figure 3 A is a graphic reconstruction of 

 the tubules shown in cross section in figure 26. In this section 

 the rows of nuclei belonging to the two tubules involved are 

 distinguishable although the process of migration of the nuclei 

 to the sides of the connecting piece is clearly under way. In 

 figure 36 of a later stage from an embryo 14 mm. in length 

 (S.C. 30), all the nuclei with the exception of one have passed to 

 the sides of the connecting sfcalk. 



In following the course of the nuclei in tubule anastomosis 

 one is but tracing the movements of the cells in which they are 

 contained, for it is hardly to be considered that the nuclei shift 

 their axes within the cells, and moreover the few faint cell bound- 

 aries which may be made out show the same changes in position 

 as do the nuclei. The hepatic cells of the connecting stalk have 

 shifted through an arc of about 90° and when a lumen is estab- 

 lished through the center of the connecting stalk it is bounded at 

 least in greater part by the same cell surfaces which were presented 

 to the lumina in the original tubules. In other words, while 

 the tubule cells involved in anastomosis have shifted in position, 

 their surfaces and their axes will bear the same relation to the 

 new lumen which they did to the former one. Their long axes 

 will be at right angles to the lumen while the inner and outer 

 surfaces of the cell remain constant in both the original position 

 in the simple tubule and the later position in the anastomotic 

 segment. The polarity of the cell, in the sense of the term as 

 used by Rabl ('88, '90), is not disturbed by anastomosis. 



The lumen of the anastomosis is formed by clefts which ex- 

 tend out from the lumina of the formerly simple tubules. These 

 clefts are at first small and irregular. They pass between the 

 rather irregular borders of the radially arranged cells of the con- 



