242 STUDY OF PIG EMBRYOS. 



ject downward so as to enclose between them a distinct groove in the median ven- 

 tral line, which can be identified as the commencing anterior fissure of the cord. 

 In this groove runs a small, longitudinal blood-vessel, the arteria sulci, which from 

 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 owing 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, 

 nevertheless, 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 vertebra, Vert, is distinctly cartilaginous, though not yet 

 fully differentiated, and is surrounded by a distinct fibrous layer, the perichbn- 

 drium. In the median line below the vertebra lie the oesophagus, CE, and tra- 

 chea, Tra, both tubes lined by entoderm. The cavity of the oesophagus is some- 

 what 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 oesoph- 

 agus. In section the sympathetic is round and contains numerous nerve-fibers 



