THE MESOMERE (INTERMEDIATE CELL MASS) 223 



tain direct connections with venous capillaries of the kidneys. The 

 outer tubule aids in forming Bowman's capsule around each glomeru- 

 lus. The ciliated nephrostoine then arises along with the mesonephric 

 unit, develops a normal coclomic connection, and then shifts its re- 

 lationship to become associated with blood sinuses of the posterior 

 cardinal vein. This occurs within the kidney, at about the 15 mm. 

 stage. The original nephrostomes remain in the adult frog on the 

 ventral face of the kidney as ciliated peritoneal funnels which are 

 readily seen. These convey coelomic fluids directly into the blood 

 sinuses of the kidney. They might still be regarded as accessory excre- 

 tory structures. More nephrostomes appear than can be accounted for 

 in the above manner and it is not known whether the extra ones arise 

 by splitting of the original ones or by additional peritoneal invagina- 

 tions to the mesonephric tubules. 



The mesonephric tubules which remain to give rise to the urinifer- 

 ous tubules of the functional and adult mesonephric kidney are elon- 

 gated considerably. They form spherical masses around developing 

 blood capillaries emanating from both the renal artery (from the 

 dorsal aorta) and from the renal portal vein. These form capillary 

 networks which are both arterial and venous and constitute true 

 glomeruli, having no connection with the coelom as do the glomi 

 of the pronephric level. Each glomerulus is surrounded by the thin- 

 walled Bowman's capsule of the fully formed Malpighian body (renal 

 corpuscle) within the mesonephric kidney. By the time of metamor- 

 phosis this mesonephric kidney is fully formed and functional, ready 

 to assume the increased excretory load of a terrestrial organism. 



The Adrenal Glands. 



Most of the endocrine glands are derived from the pharyngeal 

 endoderm or brain ectoderm, but the adrenals are derived from both 

 ectoderm and mesoderm. They are a complex of two endocrine glands 

 and are not related functionally to the excretory system but can be 

 described conveniently at this time because of their proximity of 

 origin and their final position in the adult. In the adult frog this com- 

 posite gland appears as a thin layer of yellowish brown granular 

 material on the ventral face and closely adherent to each kidney. 

 It is composed of both a medullary (suprarenal) substance and a 

 cortical (interrenal) substance, so called because of their positional 

 relationship in the higher forms rather than in the frog. 



