254 



STUDY OF PIG EMBRYOS. 



It is important to note that they do not correspond to complete subdivisions, and 

 have not the same morphological value as the three primary vesicles. The histo- 

 logical development is much less advanced than in the pig of 12 mm. The ecto- 

 derm is very thin, consisting for the most part of a single layer, of cells, but here 

 and there the formation of a second layer is seen to be beginning. The mesoderm 

 is very simple in character and almost uniform in appearance, but there is a dis- 

 tinct difference between the mesenchyma around the brain and that underneath the 

 epidermis, the former having cells farther apart. This is almost the first stage in 



the differentiation of the arachnoid zone around the 



Md.. - x""*^jsiw , ; brain. The pia mater, however, though quite thin, 



is well defined by the condensation of the 

 mesenchymaF 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 



Dien. J :. '^i-tliiB ^^&: ; ';'.' .-0 1 distinctly formed, but elsewhere has hardly begun 



its differentiation. On the inside of the medullary 

 wall, close to the surface, there are everywhere 

 very numerous mitbtic figures. 



Frontal Section through the Umbilical Opening 

 (Fig. 186). The illustration is part of the same 

 section in the series from which figure 185 is taken. 

 For convenience of comparison the position has 

 been reversed so as to bring the dorsal side of 

 the embryo uppermost in figure 186. It results 

 from this that right and left ' sides of the embryo 

 FIG. 185. PIG, 9.0 MM. FRONTAL are reversed in the engraving as compared with 

 SERIES 54, SECTION 194. the other figures of transverse and frontal sections. 



Dien, Diencephalon. M.B, Mid-brain. By examining figure 166 (pig, io mm.) the student 



Md, Medullary wall of brain, mes, .., 



,, D T> u i will see that sections in the frontal plane, owing to 



Mesenchyma. Pros, Prosencephafon. 



"v, Vein, x 22 diams. the curvature of the posterior end- of the body- wall, 



furnish transverse sections of the spinal cord of the 



pelvic region. Therefore, the section 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 beginning to change into muscle-fibers. On the dorsal side of the plate we 

 find its growing edge, m.pl, where the tissue of the muscle-plate proper bends 

 over and passes continuously into the external wall of the somite. From this 

 growing edge the cells are added to the muscle-plate by which it extends upward. 



Ve. 



Mes. 



Pros. 



