204 STUDY OF PLG EMBRYOS. 



epithelium of the tubules. Occasionally a small amount of mesenchyma can be 

 found between the tubules, or between the tubules and the nearest endothelium. 

 We have, accordingly, in these organs a typical sinusoidal circulation. The 

 blood spaces of the Wolffian body really belong to the posterior cardinal veins 

 into which the Wolffian tubules in the course of their development have, as it 

 were, penetrated, although without destroying the continuity of the vascular 

 endothelium. It is by the intercrescence of the tubules and of the endothelium 

 that the sinusoidal condition is established. A portion of the original channel 

 remains on the dorsal side of the Wolffian body, more or less free, V. card. We 

 thus learn that, owing to the development of the Wolffian body, the posterior 

 Cardinal veins as such disappear. The Wolffian duct is always on the ventral side 

 of the organ, and can easily be traced through as a continuous tube from section 

 to section. In the figure it may be easily found in the left mesonephros, it being 

 there the lowermost of the cavities drawn in the organ. On the median lower sur- 

 face of the Wolffian body, underneath the glomeruli, is an accumulation of tissue, 

 Gen, the anlage of the genital gland, which is yet very slightly advanced. Below 

 the aorta on the right side of the embryo is a large trunk of the vena cava inferior, 

 V. C. I, on its way past the right dorsal lobe of the liver. Near the aorta on the 

 left is the mesogastrium, Om. maj, or future great omentum, by which the stom- 

 ach is suspended from the median dorsal wall or the abdomen. The stomach, Si, 

 is entirely upon the left side of the body and is directly connected, by means of 

 the anlage of the lesser omentum, Om. min, with the liver. The walls of the 

 stomach are constituted by the splanchnopleure, and, therefore, comprise a 

 layer of thickened entoderm, which bounds the cavity of the organ, and a rela- 

 tively thick layer of mesoderm which forms the greater part of the wall, and the 

 very thin superficial mesothelium. The entoderm is a smooth layer of moderate 

 thickness composed of elongated epithelial cells. It forms no folds and shows no 

 trace of differentiation into gastric glands. In the mesenchyma there are some 

 capillary blood-vessels. The mesothelium is thicker than over the liver and 

 somatopleure, and contains crowded, more or less nearly spherical nuclei. The 

 liver is by far the largest organ of the body. It takes up nearly half the section. 

 It is divided into four main lobes, the two dorsal and two ventral ; two on the 

 right and two on the left. The reference line, Li, runs to the left dorsal lobe'. 

 The liver consists of a complicated network of relatively large blood sinusoids, 

 the spaces between which are entirely occupied by the embryonic liver cells. Near 

 the median plane, between the ventral lobes, appears the gall-bladder, G. bl, 

 which is cut three times. The liver is attached in the median ventral line to the 

 body- wall and to the base of the umbilical cord. The two umbilical veins (com- 

 pare Fig. 132) enter the liver directly from the cord. The veins are originally 



