HISTOGENESIS OF THE LIVER 279 
The mesothelial tubules do not remain long as such. The 
lumina are soon obliterated and the solid cords thus formed 
break up into mesenchymal strands. This process begins at 
the inner end of the cords and can be seen in figure 9. By the 
time the embryo reaches a length of 23 to 25 mm. the cords are 
entirely replaced by such mesenchymal strands which run in 
among the tubules which have now invaded this region (fig. 38). 
At 28 to 30 mm. even these strands have entirely disappeared 
and mesenchymal cells can be found only occasionally. The 
formation of mesenchyma by tubular ingrowths of mesothelium 
does not preclude the occasional delamination of single cells 
from the splanchnic mesoderm direct, but this delamination 
contributes but little to the total of the hepatic mesenchyma. 
The nuclei of the splanchnic mesothelium in embryos 5 or 6 mm. 
in length, before mesenchymal delamination begins, are broadly 
oval in outline. They have the typical structure which is found 
in the nuclei of most of the tissues at this time. Each nucleus 
contains two or three large masses staining black with Heiden- 
hain's hematoxylin. Each of these masses consists of a nucleolus 
surrounded by a thick layer of chromatin. The remainder of 
the nucleus is filled with a clear karyoplasm with a few faint 
strands of chromatin. These nuclei have been well described by 
McGill ('10) who studied the mesenchyma of the foregut region 
in the dogfish. As cells are delaminated from the mesothelium 
and as the mesothelial tubules are broken up into a mesenchymal 
syncytium, most of the nucleoli disappear and the chromatin 
is gradually scattered in a finely divided network. Similar 
changes take place in the nuclei of mesothelial cells but at a later 
period. A comparison of figures 7 and 38 will show the two 
extremes of this change. Mitotic figures are numerous in the 
mesothelium but scarce in the mesenchyma derived from it. 
The relation of the mesothelial tubules to the sinusoids is of 
interest in the light of Bremer's ('14) recent work on the earliest 
blood vessels in man. Bremer found mesothelial cords from the 
extra-embryonic coelome connected with the angiocysts and 
solid cords of the vascular net in the body stalk, and he suggests 
that the elements of the vascular net arise from these ingrowths. 
