270 STUDY OF PIG EMBRYOS. 



lateral walls are quite thick, the median ventral wall is thinner, and the median 

 dorsal wall (deck-plate) is very thin. The three primitive layers of the medullary 

 tube are very clearly marked out, the ectoglia appearing light, the ependymal layer 

 appearing dark. The differentiation is much more advanced on the ventral side 

 of the spinal cord than on the dorsal side, and, indeed, it is only in the ventral 

 part that the three layers are perfectly differentiated. In the median ventral line 

 we have the floor-plate, in which we can distinguish only two zones, while in the 

 deck-plate there is no differentiation of layers whatever. The spinal cord is clearly 

 divided into a dorsal zone, D.Z, and a ventral zone, V.Z, on each side. The two 

 dorsal zones are connected across the median line by the thin deck-plate, and the 

 ventral zones similarly by the thin floor-plate. The lower or ventral limit of the 

 dorsal zone is marked by the entrance of the dorsal or ganglionic root and by 

 the fibers, which represent the outgoing lateral roots. In the actual section figured, 

 the lateral roots, L.R.n, are those which enter into the formation of the eleventh 

 icrve. The true dorsal root does not appear in the figure. Internally the division 

 Between the two zones is marked by the lateral angle of the central cavity shown 

 n the section. In the dorsal zone the differentiation of the three layers has made 

 slight progress. In the ventral zone, however, the development is far more ad- 

 vanced. The most characteristic feature of this movement is the growth of the 

 inerea or neurone layer, which increases in a twofold manner: first, by encroach- 

 ng upon the inner or ependymal layer; and, second, by the growth of its con- 

 tituent elements. Examination with a high power shows at once that the cells 

 tave grown very much. Their nuclei are larger, granular in appearance, rarely 

 /ith any indication of a distinct nucleolus. Most of the cells are neuroblasts and 

 ave well-marked protoplasmic bodies, finely granular in texture. They have -many 

 f them already produced long, slender outgrowths which we can identify as the 

 buraxons. T" order to study the distribution of the neuraxons and the form of 

 ae ne 1 Tibia ii ;., necessary to apply the Golgi rapid method, by which it can 

 iiat a portion of the neuraxons is distributed entirely within the 

 liar iiile another portion passes out to form ventral roots, one of which, 



[".12, forming part of the hypoglossal nerve, is shown in the figure. A third 

 m of the neuraxons, at least in the upper cervical region, as also in the 

 edulla oblongata, passes' out to form the lateral roots. The positions of the exits 

 these two bundles of nerve-fibers are constant and characteristic. The ventral 

 ot always passes- out from the middle of the ventral zone about half-way be- 

 een the median floor-plate and the dorsal limit of the zone. The lateral root 

 vays passes out at the u]_^er dorsal limit of the ventral zone and immediately 

 ow the point of enhance trf" the true dorsal root. Formerly the lateral roots were 

 distinguished from\the dorsal roots. Following downward in the figure we 

 V section of the cardinal vein, Card, just inside of which lies the common 

 T , of the un^ed tenth and eleventh, or vagus and accessorius nerves, 

 the lower, part of the petrosal ganglion, N.g.petr, of the glosso- 



