280 STUDY OF PIG EMBRYOS. 



the muscle-plates, but which have now broken apart and are developing singly into 

 muscle-fibers. In certain amphibia the cells from the muscle-plate can be distin- 

 guished from the mesenchymal cells of the limb, and what we know of the devel- 

 opment of the muscles in amniota confirms the view that striated muscles and mesen- 

 chyma are genetically entirely distinct. No skeletal elements whatever have yet arisen 

 in the limb. We have here a striking illustration of the fact that the skeleton is 

 very late in its development, and, embryologically speaking, is in no sense the frame- 

 work upon which the body is built up, but rather a late supplementary develop- 

 ment. The main morphological features 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 195, but is both larger 

 and more differentiated. On the left side of the embryo the fundamental morpho- 

 logical 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 

 K 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 ven- 

 tral 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 subdivides 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 

 dowttWP^d and curves into the limb. Owing to this curvature, in order to follow 

 its cayrse the nerve must be traced through adjacent sections. If this is done, the 

 venial ramus will be found to take part in the formation of the brachial plexus. 

 Some- distance below the spinal cord is the small notochord. Farther 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 ulti- 

 mate differentiation of the submucous and muscular coats of the oesophagus and 

 trachea. The \entoderm of both the oesophagus and trachea is a moderately thick 

 layer composed of elongated cells, the nuclei of which are distributed at various 

 levels, but so as to leave the superficial portion of the layer comparatively free. It 

 in to.is superficial portion that the mitotic figures always occur. On the ven- 



