1938 



HUMAN ANATOMY. 



Right 

 umbilical artery 



Allantoic 

 duct 



Gut-tube 



Neural 

 tube 



Cloaca 



Notochord 



outgrowths from the primitive renal peKis ; according to the other, the convoluted 

 tubules (from their beginning in the capsule to their termination in the collecting 

 tubules within the medullary ray) arise independently within the renal blastema, 

 and, secondarily, unite with the duct-system from the pehis to complete the canals. 

 The careful studies and reconstructions of Huber^ leave little doubt as to the cor- 

 rectness of the latter view, which. 

 Fig. 1644. moreover, accords with the prin- 



ciple observed in the develop- 

 ment of the pronephros and the 

 Wolfifian body, in which the tu- 

 bules and the duct join subse- 

 quent to an independent forma- 

 tion. The attenuated proximal 

 end of the convoluted tubule — for 

 a short time solid and in close rela- 

 tion with the anlage of the glom- 

 erulus — soon becomes a sickle-like 

 process which gradually incom- 

 pletely surrounds the vascular tuft 

 and later expands into the charac- 

 teristic capsule. With the con- 

 tinued growth of the tubules their 

 tortuosity becomes more marked, 

 the loop of Henle early becoming 

 a conspicuous feature of their course. By the third month the formation and group- 

 ing of the tubules have progressed to such extent that the surface of the young 

 kidney exhibits the outlines of the individual lobes composing the organ. This lobu- 

 lation is retained until some months after birth. In addition to the convoluted tubules, 

 the vascular and supporting tissues are derived from the renal blastema, the con- 

 densed peripheral part of which becomes the fibrous capsule of the kidney. As the 

 latter assumes the role of active excretory organ, the Wolfifian body undergoes atrophy, 

 with the exception of such parts as are concerned in the development of the sexual 

 ducts. 



The Bladder and the Urethra. — The details of the development of the 

 bladder and urethra in mammals and man have been materially advanced by the 



Cloacal membrane 



End-gut 



Tail-bud 



Reconstruction of caudal portion of human embr\o of seven- 

 teen days (3 mm. greatest length), showingcloaca connected 

 with gui and allantoic duct. X 48. {Drawn ft om Keibel model.) 



Allantoic duct 



Bladder 

 segment 



Fig. 1645. 



Gut-tube f 



Wolffian duct 

 Renal bud 



Fig. 1646. 

 Gut-tube . 



Cloacal membrane. 



Ventral segment of 

 cloaca 



Dorsa) segment of cloaca 



Allantoic duct 



Bladder 



segment 

 Septal fold 



Cloacal membrane 



Reconst;uction of cloacal region of human 

 embryo of twenty-six days (6.5 mm. length) ; 

 Wolffian duct opens into ventral segment of 

 clopca. X 75. i, Drawn from Keibel model.) 



Preceding model viewed from right side, show- 

 ing laeginning division of cloaca into ventral (uro- 

 genital) and dorsal (intestinal) segment by longi- 

 tudinal septal fold. {Drawn from Keibel model?) 



investigations of Keibel. Retterer, and Nagel, upon whose conclusions the following 



account is based. A sagittal section through the caudal pole of an early human 



embryo of 6.5 mm., about the beginning of the fourth week (Fig. 1645), exhibits 



'American Journal of Anatomy, vol. iv.. Supplement, 1905. 



