HISTOGENESIS OF THE LIVER 295 



constricted proximal necks. Secondary tubules of the second, 

 third or fourth order may be produced from the terminal expanded 

 chamber before tubule anastomoses are formed. 



4. In Acanthias the hepatic tubules may differentiate in regions 

 into which blood bessels have not penetrated. 



5. Tubule anastomosis and the complete establishment of 

 sinusoidal circulation are practically simultaneous. 



6. Anastomoses of tubules may take place end to end, side 

 to end, or side to side. The frequency of the several types is 

 in the order stated. 



7. The process of anastomosis involves the stages of (a) the 

 contact and fusion of the tubules involved, (b) the rearrangement 

 of cells in the area of confluence, and (c) the establishment of a 

 new connecting lumen. 



8. While the cell position is shifted in anastomosis, the cell 

 axis remains unchanged. 



9. After anastomosis the increase in the number of hepatic 

 cylinders takes place in three ways, viz: (a) the formation of he- 

 patic cylinders directly from the pouch wall; (b) the inter- 

 stitial development of cylinders from blind sprouts from the net- 

 work; (c) the peripheral growth of the terminal hepatic cjdinders. 



10. These methods of increase cease in the order named and 

 contribute to the network inversely to the order given above. 



11. Increase in the number of cylinders of the hepatic network 

 firet ceases in the body of the median lobe and last in the tips of 

 the lateral lobes and along the dorsal margins of the lateral lobes 

 and connecting portion. 



12. The number of cells surrounding the lumen of the hepatic 

 cylinder in cross-section drops from an average of 12 + in the 

 tubule at the time of anastomosis (14 to 15 mm.) to 3 ± in an 

 embryo 95 mm. long. 



13. The diameter of the hepatic cylinders increases up to and 

 immediately after the time of anastomosis. With the enormous 

 increase in the size of the sinusoids following a short time after 

 anastomosis, the diameter for a time decreases. Thereafter 

 the diameter of the cylinders undergoes a steady increase at least 

 to the time of birth or hatching. 



