196 STUDY OF PIG EMBRYOS. 



tration of the fact that the skeleton is very late in its development, and, embryo- 

 logically speaking, is in no sense the framework upon which the body is built up, 

 but rather a late supplementary development. The main morphological fea- 

 tures in all parts of the embryo are entirely fixed by the soft tissues before the 

 skeletal structures arise. Both nerves and blood-vessels have grown into the 

 limb. The nerves are the ventral branches of the spinal nerves. Several of 

 these unite together and form the brachial plexus, one part of which, Br. plx, is - 

 shown in the section. In the present embryo this nerve-trunk includes fibers 

 derived from both the sixth and seventh cervical nerves. Just above the nerve- 

 trunk is the section of the subclavian or axillary vein, which is a branch from the 

 jugular. The dorsal region of the embryo is relatively larger at the level of this 

 section than higher up, owing chiefly to the great development of the mesoderm. 

 The spinal cord, Sp. c, resembles that in figure 1 19, but is both larger and more 

 differentiated. On the left side of the embryo the fundamental morphological 

 characteristics of the spinal nerve are well illustrated in this section. The dorsal 

 root, D. R, bears the ganglion, G, which joins the dorsal zone of the spinal cord. 

 The fibers of this root are produced from the cells of the ganglion and grow from 

 the ganglion into the spinal cord. Other fibers from the same cells grow out in 

 the opposite direction and form the nerve-trunk or root which descends from the 

 ganglion in a nearly vertical direction. The ventral root, V. R, arises from the 

 ventral zone, takes an oblique course, and joins the dorsal root a little below the 

 level of the spinal cord to form a single nerve-trunk, which, however, soon sub- 

 divides into its two primary branches. The first or dorsal branch, R.D, bends at 

 an acute angle upward and outward. The second or ventral branch, ramus 

 ventralis, continues downward and curves into the limb. Owing to this curva- 

 ture, in order to follow its course the nerve must be traced through adjacent 

 sections. If this is done, the ventral ramus will be found to take part in the 

 formation of the brachial plexus. Some distance below the spinal cord is the 

 small notochord. Further down, but also in the median line, appear two small 

 rings of epithelium. Of these, the smaller upper one, (E, is the entodermal lining 

 of the oesophagus, and the larger lower one is the entodermal lining of the trachea. 

 Around each of these rings there has already occurred a slight condensation of 

 the mesenchyma, the first step toward the ultimate differentiation of the sub- 

 mucous and muscular coats of the oesophagus and trachea. The entoderm of 

 both the oesophagus and trachea is a moderately thick layer composed of elon- 

 gated cells, the nuclei of which are distributed at various levels, but so as to 

 leave the superficial portion of the layer comparatively free. It is in this super- 

 ficial portion that the mitotic figures always occur. On the ventral side of the 

 trachea and quite close to it appear two small blood-vessels, the pulmonary arte- 



