174 



LIVEE 



;; 



or 



septum transversum, the mesenchyme of which supplies the capsule and co 

 nective -tissue framework of the organ. As we have already seen, the omphal 

 mesenteric and allantoic veins pass to the sinus venosus through this septum, 

 and we now see the growing liver-parenchyma in the form of cellular strands 

 trabeculae extending round these vessels. The cells of the trabeculae continue to 

 proliferate very rapidly, and the buds produced invade the lumen of the vessels, 

 pushing the endothelial walls before them (figs. 218, 219). This process goes on until 

 the course of the vessels is completely interrupted, and the original channels remain 

 merely as capillary-like vessels, named by Minot ' sinusoids.' l The ultimate result 

 is a system of anastomosing epithelial trabeculae, in the meshes of which there is a 



FIG. 220. TBANSVEBSE SECTION OP A HUMAN EMBBYO AT THE END OF THE FIFTH WEEK. 

 Photograph. (T. H. Bryce.) 



s.g., s.g., spinal ganglia, neural cartilages to their outer sides ; s.n., spinal nerve ; not., notochord in 

 vertebral body ; sy, sympathetic ganglion ; ao, aorta ; Ig, lung ; ce, oesophagus ; m.p.p., membrana 

 pleuroperitonalis ; H, liver ; B, rib. 



second network of sinusoids. The thick trabeculse of early stages are gradually 

 reduced to columns in which only a single layer of cells bounds the narrow lumen, 

 which has meanwhile appeared in the solid strands. The slit-like lumen becomes 

 the bile-capillary. From the second month onwards during the earlier months 

 of pregnancy the liver is very actively engaged as a blood-forming organ, the 

 vessels being crowded by young nucleated red blood-corpuscles. 



Though the liver-parenchyma is laid down in higher forms as solid anastomosing strands, 

 in some lower vertebrates (Cyclostomata, Selachians, Amphibians, and certain Reptilia) the 



Minot, Procj Boston Soc. Nat. Hist. xxix. N. 10, p. 185. 





