308 STUDY OF PIG EMBRYOS. 



glomerulus of the mesonephros. Each glomerulus has a capillary circulation, 

 and the blood on leaving the glomerulus is supposed to be emptied into the venous 

 sinusoids. More exact investigation of . this point is needed. The mesonephros 

 is covered by a layer of mesothelium, msth, underneath which is a thin layer of 

 mesenchyma. The two together constitute the anlage of the peritoneal covering 

 of the organ. To the median side of the Wolffian body is appended the large 

 anlage of the genital gland, Gen, which has a constricted connection with the 

 Wolffian body. Each gland is covered by mesothelium and extends until it comes 

 in contact with the mesentery, mst. The gland contains two kinds of tissue, one, 

 the anlage of the medullary, the other of the cortical portion of the gland. The 

 medullary tissue resembles the neighboring mesenchyma and occupies only a small 

 territory about the stalk of the organ. The cortical tissue contains cells with much 

 larger nuclei and clearly developed protoplasmic bodies. It occupies by far the 

 larger part of the gland. Comparison with figure 198 will show that the genital 

 anlage at this stage occupies -the same topographical relation to the Wolffian body 

 as at earlier stages. It differs now from the earlier condition chiefly by its growth 

 in size and by its advancement in histological differentiation. Below the genital 

 gland the intestinal canal is cut several times. One portion of the intestine is 

 seen in the section to be connected by means of the mesentery, mst, with the 

 median dorsal tissues of the embryo. The intestine is formed by a small tube 

 of entoderm with a small cavity. The entoderm is a rather thick cylinder epithe- 

 lium. The greater part in bulk of the walls of the intestine is constituted by 

 mesenchyma. The external surface is covered by a thin mesothelial layer. The 

 mesenchyma is beginning to show the differentiation of the external muscular 

 from the internal mucous coat. There is at this stage no trace whatever of the 

 development of any folds or glands on the inside of the intestinal canal. 



Section through the Kidney (Fig. 210). This section being much nearer the 

 caudal end of the embryo, we find, as throughout all the early stages, that the 

 differentiation of the tissues is less advanced than nearer the head. We have 

 accordingly, so to speak, an earlier stage in the development of the spinal cord, 

 Sp.c, -of the nerves, and of the vertebra. In the median line is the large aorta, 

 Ao, about which the mesenchyma is only slightly condensed. Near the aorta are 

 the conspicuous anlages of the sympathetic system, Sym, which appear at this level 

 in a very characteristic hook-shaped pattern. At the dorsal end of the hook the 

 nerve-fibers are much more numerous than in the ventral portion of the anlage. 

 The sympathetic cells themselves are extremely conspicuous, owing to^ the depth of 

 their stain. On either side is situated the anlage of the permanent kidney, Ki. 

 Each anlage consists of an irregularly branching space bounded by a thick layer 

 of epithelium, which has somewhat the appearance of the intestinal entoderm at 

 this stage. If the series of sections be followed through farther toward the tail 

 of the embryo, the epithelial space will be seen to contract to a relatively small 

 tube, the ureter, which opens into the Wolffian duct of the same side. The ex- 



