EMBRYO OF 17 MM. 237 



pies 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 122 will 

 show that the genital anlage at this stage occupies the same topographical rela- 

 tion 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 epithelium. 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 differentia- 

 tion 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. — 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. 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 further 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- 

 panded portion of the cavity shown in our figure corresponds in part to the pelvis 

 of the adult organ. Its irregular shape is due to the fact that it is forming a 

 series of outgrowths, which are to give rise to the tubules of the kidney. Around 

 the ends of the branches of the renal pelvis is a darker tissue, in which the cells 

 are very much crowded. The nature of this tissue has been much debated. Two 

 divergent interpretations of it have been offered. According to one view, it is the 

 material out of which the glomeruli and convoluted tubules of the kidney are to 



