THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 



397 



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. 276). Frequently the two masses fuse across the medial line 

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



Cortex 

 Connective tissue 



Medulla 

 Cortex 



Cortex 



Medulla 

 (Phaeochrome cells) 



FIG. 350. 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 inter renal 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 



j sympathetic ganglion cells, and (2) phaochromoblasts which are destined to give 



| rise to the phaeochome or chromafiin cells (Fig. 349). Hence the chromaffin 



i 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 



