FRONTAL SECTIONS OF EMBRYO OF 12 MM. 301 



of the hepatic veins with the main vessel are cut in the section. The liver is 

 attached to the diaphragm. Above the diaphragm the cava is continued, with thin 

 walls, for a short stretch, S.V, which is the modified sinus venosus of the heacrt, 

 and which opens directly into the right auricle, Au.D. The opening is guarded 

 by two valves, the valvula sinistra, V.S, on the left, and the valvula Eustachii, 

 V.E, on the right, which together prevent the back-flow of the blood !rom~THe r 

 heart into the vein. Above the heart appear the pulmonary aorta, P. A, and the 

 two descending aortae, Ao.D, Ao.S. The main dorsal aorta, Ao, shows in the 

 lower part of the section. The stomach, St, lies on .the left side and is closely 

 attached to the liver by the short and thick anlage of the great omentum, and is 

 attached to the caval ligament .by the longer band of the lesser omentum, Om. 

 min. The space bounded by the stomach, the lesser omentum, and the liver is 

 the lesser peritoneal cavity (bursa omentalis). In the Wolffian body the sub-cardinal 

 vein, V.s.c, is easily identified, and with a higher power the intertubular sinu- 

 soids reveal their characteristics clearly, the sinusoidal epithelium being fitted closely 

 to the surface of the Wolffian tubules. The division of the ccelom by the dia- 

 phragm into an upper pericardial and a lower abdominal chamber is perfectly demon- 

 strated by this section. The student should observe that the mesothelium forms 

 for both chambers the absolutely unbroken boundary of the ccelom. 



Section through the Dorsal Vertebra (Fig. 207). Owing to the curvature of the 

 embryo the spinal cord is cut twice; once, Sp.c', toward the head end of the 

 embryo, and again, Sp.c", lower down toward the tail end. Alongside the sections 

 of the spinal cord appear the large, darkly stained masses of the ganglia, G. The 

 section also passes through the bases of the anterior limbs, A.L, in one of whi 

 can see one of the branches, N.br, of the brachial plexus. Between the wo | 

 of the spinal cord of the section the plane passes on the ventral side of 

 cord and shows the series of vertebral formations, together with the nc 

 N',N",- the intersegmental arteries, A.i.s, and the segmental veins, small \essds 

 which lie close to the intersegmental arteries. The nerves are sections of the 

 dorsal root below the ganglia. Each nerve has a distinct outline and is partly 

 penetrated by ingrowing mesenchymal cells which subdivide the nerve into rounded 

 fiber bundles. In each bundle the nerve-fibers appear as fine dots, which, how- 

 ever, by the use of the fine adjustment can be followed up and down through 

 the section, and thus identified as fibers. The single fibers are more or 

 less isolated from one another, and between them are delicate threads, the nature 

 of which is not known. Between the adjacent rounded bundles of fibers there 

 is often a distinct space. The anlages of the intervertebral disc, fv.D, are 

 formed entirely from condensed mesenchyma, and therefore stand out somewhat 

 conspicuously in the section owing to their darker staining. Each anlage is bow- 

 shaped, the concavity of the bow facing toward the tail of the embryo. The end^ 

 of the bow pass behind the nerve-trunk of the segment to which the anlage be- 

 longs. The anlages extend completely across the median line, and by following 



