402 TEXT-BOOK OF EMBRYOLOGY. 



straight tubules. This is the beginning of the renal cortex. A true cortex, 

 however, can be spoken of only after the appearance of the glomeruli (in 

 embryos of 30 mm.). Its peripheral boundary is the capsule, and the renal 

 corpuscles nearest the pelvis mark its inner boundary. The mass of straight 

 tubules forms the bulk of the medulla. It does not at this stage contain Henle's 

 loops, the latter developing later (during the fourth month). Both cortex 

 and medulla increase until the kidney reaches its adult size. The cortex 

 increases relatively faster than the medulla up to the seventh year; after 

 this the increase is practically equal. The medullary rays are probably 

 secondary formations, being formed by groups of straight tubules which 

 grow out into the cortex; later, ascending arms of Henle's loops are added to 

 these groups. 



Some of the glomeruli of the first generation are much larger than any 

 found in the adult. In some of the lower Mammals these "giant" glomeruli 

 disappear and it is probable that the same occurs in the human embryo. Some 

 of the tubules also degenerate and disappear. The cause of these phenomena 

 is not known. 



Changes in the Position of the Kidneys. — As has already been described 

 (p. 394), the kidney buds first grow dorsally from the mesonephric ducts 

 toward the vertebral column. They then grow cranially, with a corresponding 

 elongation of the ureters, and in embryos of 20 mm. they lie for the most part 

 cranial to the common iliac arteries. This migration continues until the time 

 of birth when the cephalic ends of both kidneys reach the eleventh thoracic ver- 

 tebra. When the kidneys begin to move cranially the hilus is directed caudally. 

 Later they rotate and the hilus is turned toward the medial sagittal plane. 



Since the ureter, renal pelvis and straight tubules develop from the mesonephric ducts, 

 and since the convoluted tubules and glomeruli develop directly from the same tissue as the 

 mesonephric tubules, namely, the mesenchyme, the renal tubules may be said to represent 

 the third generation of urinary tubules. But no definite reason for the appearance of the 

 third generation can be given. The atrophy of the mesonephroi would, of course, make 

 necessary the compensatory development of new structures; but this only carries the problem 

 a step further back, for the cause of the atrophy of the mesonephroi is not clear. In regard 

 to this atrophy, however, there is a suggestion of a cause in the fact that in the Amphibia 

 the mesonephroi are in part used for conveying the sexual elements, which leaves the meso- 

 nephroi less free to function as urinary organs. Possibly the loss of freedom to function leads 

 to the development of new structures — the kidneys — in the higher forms (Reptiles, Birds 

 and Mammals). In these forms the kidneys assume the urinary function after the early 

 embryonic stages, and only the ducts and a part of the tubules of the mesonephroi persist in 

 the male to convey the sexual elements. Thus the persistent parts of the mesonephroi as- 

 sume a new function as the old one is lost. But, on the other hand, complications arise 

 on account of the fact that in the female the sexual products are carried off by another set 

 of ducts (the Mullerian ducts), which develop in both sexes but disappear in the male, 

 while the mesonephroi and their ducts disappear almost entirely. 



