THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 



427 



precision during the development of an individual : The two parts arise sepa- 

 rately, come closer together, and finally unite. 



The Cortical Substance. The cortex is of mesothelial (mesodermal) 

 origin. In embryos of five to six mm. the mesothelium at the level of the 

 cephalic third of the mesonephros proliferates and sends buds or sprouts into 

 the mesenchyme at each side of the root of the dorsal mesentery. These 

 sprouts soon lose their connection with the parent mesothelium and unite with 

 one another to form a rather compact mass of epithelial-like cells ventro-lateral 

 to the aorta (Fig. 314). Frequently the two masses fuse across the medial line 

 ventral to the aorta. They constitute the anlagen of the cortical substance of 





Connective tissue MS 



Cortex 



Medulla 

 (Phaeochrome cells) 



FIG. 388. From a transverse section of a 40 mm. pig embryo, showing the growth of the medullary 

 substance into the cortical substance of the suprarenal gland. The vessel in the center of 

 the figure is the aorta. Wiesel. 



the two suprarenal glands. From the fact that in the lower forms they remain 

 separate from the medullary substance and lie between the urinary organs, 

 they are known as the interrenal organs. 



The Medullary Substance. A little later than the appearance of the 

 cortical anlage, the cells of some of the developing sympathetic ganglia become 

 differentiated into two types (i) the so-called sympathoblasts which develop into 

 sympathetic ganglion cells, and (2) ph&ochromoblasts which are destined to give 

 rise to the phceochome or chromafiin cells (Fig. 387). Hence the chromafrin 

 cells are derivatives of the ectoderm, since the ganglia are of ectodermal origin. 

 They soon become more or less separated from the ganglia, migrate to the 



