234 STUDY OF PIG EMBRYOS. 



chymal cells, characterized by the greater development of their protoplasm and 

 by their elongated form. Traces of the differentiation of the outer layer into 

 the inner circular muscular coat and the outer longitudinal coat of the adult are 

 clear in the section. The spinal cord, Sp. c, has changed its outline in section, 

 being broadest in the ventral zones, which have also begun to expand ventral- 

 wards so that the outline of the cord shows on its inferior side a concavity, the 

 first indication of the ventral fissure. The three layers of the spinal cord are 

 very distinct. The change in form, however, it can be clearly seen, is due 

 chiefly to the growth of the gray layer, cin, especially in the ventral zone. The 

 gray layer in the dorsal zone is still very slightly developed. From the dorsal 

 zone descends on either side the dorsal nerve-root, D. R, which presently joins 

 the ganglion, G. The ganglion now occupies a much lower position than in the 

 earlier stages (compare Fig. 122, G). From the ventral zone springs the ventral 

 root which unites with the dorsal at the lower tip of the ganglion. From the 

 nerve-trunk thus formed there is given off almost immediately the dorsal branch , 

 R. D, which soon ramifies in the midst of a dark mass of tissue, the anlage of the 

 dorsal musculature. The main nerve-trunk descends ventralwards and sends off 

 at the level of the vertebra a sympathetic branch, R. sym, which runs obliquely 

 downward and inward toward the aorta, and there terminates in the anlage of the 

 sympathetic chain, Sym, which consists partly of nerve-fibers, partly of ganglion 

 cells which have migrated along the nerve and taken up their position at its end. 

 These cells are easily recognized by their very dark staining. Their nuclei are a 

 little lighter than those of the neighboring mesenchymal cells, but, owing to their 

 deep coloration, stand out conspicuously, even when the section is examined 

 only with the low power. The sympathetic anlage comes in close contact with a 

 portion of the cardinal vein, card, near the aorta. The main nerve-trunk, R. V, 

 continues obliquely downward and presently forks into an upper and a lower 

 branch. The cardinal veins, card, lie on either side of the aorta, but they are 

 almost completely obliterated by the ingrowth of the Wolffian tubules, which 

 subdivide the vein into numerous smaller channels or sinusoids. The section 

 also shows two branches, Ve' , and Ve", of the subclavian vein. The identity of 

 these branches has not yet been determined. The aorta, Ao, is a very large ves- 

 sel a little to the left of the median plane. It has a well-developed muscular 

 coat. Beneath it follows the oesophagus, the lumen of which is much smaller 

 than that of the aorta. Its epithelium has the general characteristics of the 

 epithelial entoderm at this stage, being a rather thick cylinder epithelium. As 

 above mentioned, the differentiation of the mucous and muscular layers of the 

 oesophagus shows clearly. Below the oesophagus lie the two large vagus nerves, 

 N. 10, and then follow the sections of the two lungs, Lu. Each lung is a lobe of 



