Charles R. Bardeen 265 
of the left side, the axial mesenchyme, the aorta, the left cardinal vein, 
the intestines and the uro-genital organs are not shown. A portion of 
the right cardinal vein and a portion of the right umbilical artery are 
represented, reduced in size for the sake of clearness. The umbilical 
artery curves about the distal extremity of the celom. From the um- 
bilical artery a branch passes into the limb-bud. Veins pass from the 
limb-bud into the cardinal vein. The blood-vessels of the limb exist at 
this time in the form of an irregular plexus. 
The second, third and fourth lumbar nerves may be seen sending 
spreading bundles of nerve fibres into the dense tissue of the limb, 
dorsal to the cardinal vein. They extend, however, for no considerable 
distance into the limb-bud. The myotomes end abruptly near the base 
of the hmb-bud. 
Plate II, Fig. 1, represents the tissue differentiation in a section 
through the posterior limb-buds of Embryo II. At the left the bud is 
shown cut through an area near the distal extremity of the cclom. At 
the right the cut is more dorsal and extends through the tips of the 
lumbar spinal nerves. 
In Plate I, Fig. 2, are shown the right limb and the posterior half 
of the trunk from the 26th (8th thoracic) to the 30th (5th sacral) 
spinal segments in a slightly older embryo (CLXIII, length 9 mm.). 
Bundles of nerve fibres from the five lumbar and first two sacral nerves 
have become anastomosed into a plexus from which in turn four nerves 
have sprung. These represent the femoral, obturator, tibial and per- 
oneal nerves. Within the limb the central mesenchyme, near the axis 
of the embryo, has become condensed. This condensed mesenchyme 
represents the femur and hip bone of the adult limb. In the drawing 
the outline of this sclerogenous tissue is made diagrammatically sharp. 
The femoral portion of the skeletal mass fades gradually into the un- 
differentiated mesenchyme of the distal portion of the limb. It is this 
skeletal mass which seems to divide the bundles of nerve fibres into the 
four main divisions which constitute the origin of the four chief nerves 
of the limb. The main artery and vein of the limb are represented at a 
reduced scale. The border vein at this period is well developed (see 
also Fig. C, Plate III of the article by Bardeen and Lewis, or). 
The differentiation of the tissue of the limb-bud, first noticed in a 
condensation of tissue in the region corresponding to where the femur 
projects against the hip girdle, is quickly followed by further changes. 
Externally there becomes visible a differentiation of the limb into foot- 
plate, crus and thigh, while within the limb-bud the further development 
