448 DR THOMAS H. BRYCE ON 
it descends on the left of that organ, and, as in stage 31, the cord passes into the general 
mass of the yolk cells. 
Round the cord of cells the mesenchyme forms a layer, differing from the general 
somatic mesenchyme in being composed of rounded cells closely packed together. 
Traced back, the layer sweeps over the yolk, surrounds the vessels and ducts entering 
the liver, passing with them to join the general connective tissue of that organ. 
The very anterior edge of the ventral mass of yolk cells is indicated in fig. 1. As 
we shall see later, this tissue is composed entirely of mesenchyme, and it passes into the 
liver round the portal vein, 
Behind the point where the definitive hypoblast of the solid gut joins the general 
mass of yolk cells, a mass of the mesenchyme projects free (text fig. 3) into the body 
Fie, 3.—Section of larva, stage 32+, through posterior edge of liver, behind point where the solid gut rudiment becomes 
continuous with undifferentiated yolk cells. x 38d. 
A, aorta; LL, lungs; Y, mass of undifferentiated yolk cells: M, mesenchyme on surface of mass of yolk cells; 
N, specialised tract of mesenchyme along pronephric duct. 
cavity to the left of the hinder end of the liver. On each side this fades away into the 
general mesendodermic layer of the yolk, while behind this point the splanchnic 
mesenchyme, as such, is absent (text fig. 4), until again posteriorly it surrounds the 
solid hind gut which is being differentiated from behind forwards (text fig. 5). The 
general somatic mesenchyme is at this stage formed of widely separated stellate or 
fusiform cells, but the tract adjoining and surrounding the pronephric duct, the 
pronephric funnels, and mesonephric tubules has quite special histological characters. 
This tract can be followed all along the body of the embryo. 
Before proceeding to describe the minute characters of the splanchnic mesenchyme 
and of this specialised tract of the somatic mesenchyme,* it may be observed, as a fact 
* See note, p, 464. 
