EMBRYO OF 20 MM. 251 



be followed downward past the Wolffian body, W. b' , and liver, and from there 

 past the genital gland, Gen, and so on to the lowest part of the abdominal cavity, 

 Ab. cce. The pleural cavity at this stage is entirely separated from the pericar- 

 dial, but it is still directly continuous with the abdominal cavity. On the ven- 

 tral side (in the figure, to the right) of the pleural cavity are the great veins, the 

 duct of Cuvier, D. C, descending from above, and the ductus venosus, Du. V, 

 rising from below. The pleural cavity is separated from the duct of Cuvier by a 

 lamina of the mesoderm, x, and from the ductus venosus by a similar but thinner 

 lamina, y. Both laminae are bounded on the pleural side by the mesothelium, 

 and on the venous side by the endothelium of the vessel. The opening of the 

 veins into the right auricle, Au. d, does not appear in this section, though a small 

 bit of the left valve, v. s, which guards this opening is shown. The Wolffian body 

 is divided into two parts, an upper, W . b' , on a level with the liver, and a lower, 

 W. b", toward the pelvic end of the abdomen. The lower part is larger than the 

 upper. The two parts are separated from one another chiefly by the mesone- 

 phric vein, V. mm, which is the principal vessel to take the blood from the Wolff- 

 ian body. It delivers the blood to the lower end of the vena cava inferior. The 

 separation of the two parts of the Wolffian body is, however, further accented by 

 the position of the genital gland, Gen, and of the kidney, Ki. The structure of 

 the latter organ does not differ much from that of earlier stages, except that the 

 diameter of the tubules has increased. The genital gland (testis) is remarkable 

 for its large size. It is covered by a layer of mesothelium, underneath which is 

 a rather broad layer of elongated mesenchymal cells, the anlage of the tunica 

 albuginea. The interior of the organ contains a number of contorted epithelioid 

 cords of cells in which there are a certain number of so-called primitive ova, cells 

 which are distinguished by their larger size, rounded form, greater transparency, 

 and spherical nuclei. The bands of cells are known as the sexual cords, and they 

 are separated from one another by loose mesenchymal tissue. The cords fre- 

 quently anastomose with one another. They are the solid anlages of the semi- 

 niferous tubules. The question of the origin of these cords has been much de- 

 bated, but cannot be considered as yet settled. As to both the origin and the 

 ultimate fate of the primitive ova in the gland we have no satisfactory informa- 

 tion. The cords remain solid throughout embryonic life, not acquiring a central 

 cavity until after birth. The kidney is well defined and is similar in structure to 

 the kidney in the 17 mm. pig. (page 237), but is somewhat more advanced in its 

 organization, especially as regards the formation of the glomeruli and convoluted 

 tubules. The liver is a very voluminous organ permeated everywhere by sin- 

 usoidal blood-vessels, which. offer the greatest possible variety in size. In the 

 figure only the larger of these blood-vessels have been drawn in. A large pro- 



