EMBRYO OF 9 MM. 



223 



Md. 



M. B. 



Dien . 



underneath the epidermis, the former having cells further apart. This is almost 

 the first stage in the differentiation of the arachnoid zone around the brain. The 

 pia mater, however, though quite thin, is well defined, by the condensation of the 

 mesenchymal cells and by the somewhat numerous small blood-vessels in it. 

 The medullary wall is everywhere quite thick and crowded with nuclei. In the 

 region of the diencephalon the ectoglia is distinctly formed, but elsewhere has 

 hardly begun its differentiation. On the inside of the medullary wall, close to 

 the surface, there are everywhere very numer- 

 ous mitotic figures. 



Frontal Section through the Umbilical Open- 

 ing. — The illustration (Fig. 132) is part of the 

 same section in the series from which figure 131 

 is taken. For convenience of comparison the 

 position has been reversed so as to bring the 

 dorsal side of the embryo uppermost in figure 

 132. It results from this that the right and 

 left sides of the embryo are reversed in the 

 engraving as compared with the other figures 

 of transverse and frontal sections. By exam- 

 ining figure 99 (pig, 10 mm.) the student will 

 see that sections in the frontal plane, owing to 

 the curvature of the posterior end of the body- 

 wall, furnish transverse sections of the spinal 

 cord of the pelvic region. Therefore, the sec- 

 tion here figured, although part of a frontal 

 series, is directly comparable to a transverse 

 section of the body. In the upper part of the 

 figure we have the spinal cord, Sp. c, and on one 

 side of that the ganglion, G. Owing to the 

 spiral twist of the embryo the section is not 

 symmetrical, so that the posterior limb, P. L, 



appears only on one side of the section. Laterad from the nerve shown in 

 the figure is the large muscle plate, My, the cells of which are already begin- 

 ning to change into muscle-fibers. On the dorsal side of the plate we find 

 its growing edge, in. pi, where the tissue of the muscle plate proper bends 

 over and passes continuously into the external wall of the segment. From 

 this growing edge the cells are added to the muscle plate by which it ex- 

 tends upward. The similar edge on the ventral side provides for the extension 

 of the muscle plate downward. In the median line, below the spinal cord, we 



Pros. 



Fig. 131. — Pig, 9.0 mm. Frontal 



Series 54, Section 194. 

 Dim, Diencephalon. M.B, Mid-brain. 

 Md, Medullary wall of brain, mes, 

 Mesenchyma. Pros, Prosencephalon. 

 Ve, Vein. X 22 diams. 



