314 STUDY OF PIG EMBRYOS. 



time to time gives off small branches, which enter the substance of the spinal cord. 

 In the ventral zone the ependymal layer has become quite thin and the middle or 

 gray layer has acquired great thickness, chiefly owin^ to the growth of the neuro- 

 blasts, many of which, especially toward the outside of the cord, can now be 

 readily identified as young nerve-cells. The ectoglia or outer neuroglia layer has 

 increased in thickness. Many of the processes of the neuroglia cells can be 

 readily distinguished, running, for the most part, more or less nearly perpendicular 

 to the surface of the cord. Between the neuroglia fibers are numerous fine dots 

 which are the cut ends of the nerve-fibers running longitudinally. Although about 

 these nerve-fibers there are as yet no medullary sheaths developed, it is, never- 

 theless, proper to speak now of the ectoglia as the external white matter of the 

 cord. Immediately beneath the entrance of the dorsal root the external outline 

 of the cord shows a concavity which disappears in later stages. The dorsal 

 zones are very much smaller than the ventral. The differentiation of their three 

 primary layers is being completed by the development of a distinct middle layer. 

 The ectoglia of the dorsal zone resembles that of the ventral zone in structure 

 and thickness. The spinal ganglia, G, have descended from their original position, 

 so that they now lie on a level with the lower edge of the spinal cord, and the 

 nerve-root, by which each ganglion is connected with the dorsal zone of the cord, 

 has correspondingly elongated. The lower edges of the ganglia come in contact 

 with the lateral processes of the vertebra. Between the spinal cord and the 

 vertebra is an area of loose mesenchyma which may be regarded as a portion of 

 the arachnoid membrane. Close to the upper surface of the vertebra, bounded 

 dorsally by the tissue just mentioned, are two symmetrically placed blood-vessels. 

 The intervertebral ligament, Iv.D, is only partially cut. Above it appears the 

 lighter tissue of the next following vertebra, which is shown better several sections 

 lower down. The vertebra is distinctly cartilaginous, though not yet fully differ- 

 entiated, and is surrounded by a distinct fibrous layer, the perichondrium. In 

 the median line below the vertebra lie 'the oesophagus, ffi, and trachea, Tra, both 

 tubes lined by entoderm. The cavity of the oesophagus is somewhat crescent- 

 shaped, that of the trachea triangular. About the oesophagus the mesoderm forms 

 two layers, an inner lighter layer and an outer muscular layer, the cells of which 

 are already elongated. The mesenchyma about the trachea is more condensed, 

 especially on the sides and below, and the condensed tissue is in close contact 

 with the epithelium. On the dorsal side of the trachea close to the entoderm is 

 a thin layer of transversely elongated cells. The sympathetic nervous system, Sym, 

 appears symmetrically placed near the trachea and oesophagus. In section the 

 sympathetic is round and contains numerous nerve-fibers and characteristic young 

 sympathetic nerve-cells, by which it is readily recognized. Close to the ventral 

 side of the sympathetic is the section of the large jugular vein, V.jug, a branch of 

 which, V.br, lies laterad from the main vessel. This branch receives blood-ves- 

 sels from the facial region. Between the main jugular and its branch are some 



