590 THE HUMAN EMBRYO. 



the Malpighian bodies increase greatly, both in number and in 

 size ; and new Wolffian'tubules are formed, apparently by budding 

 from the old ones. In each tubule the part next the Malpighian 

 body, which is probably the secreting portion, has thicker walls, 

 formed of larger epithelial cells, than the more distal part which 

 opens into the Wolffian duct. 



The Wolffian body reaches its greatest development about 

 the eighth week, from which time it slowly diminishes in size. 

 Degeneration commences, and proceeds more actively at the 

 anterior end of the Wolffian body, which from the first has lagged 

 behind the rest of the organ in development. Ultimately the 

 whole structure becomes affected ; by the fifth month the Mal- 

 pighian bodies have almost entirely disappeared, and in the end 

 the Wolffian body becomes reduced to an accessory part of the 

 reproductive apparatus. 



2. The Kidney and Ureter. 



The ureter arises on each side as a diverticulum from the 

 hinder end of the Wolffian duct, in the early part of the fourth 

 week (Fig. 243, kd). This soon acquires an independent 

 opening into the cloaca, a little way behind that of the Wolffian 

 duct (Figs. 216, kd, and 233, n). At its opposite or blind end 

 the ureter grows forwards, between the hinder end of the Wolffian 

 body and the vertebrae. It dilates to form a somewhat elongated 

 sac, which is the pelvis of the future kidney ; and from this 

 sac branching tubular diverticula grow out (Fig. 234, n), and 

 become the urinary tubules. These rapidly increase in number 

 and in length ; Malpighian bodies are formed in connection with 

 their distal ends, and the kidney structure is definitely acquired 

 by the end of the second month, at which time the degeneration 

 of the Wolffian body commences. 



The bladder is formed by dilatation of the basal or proximal 

 part of the allantois. Beyond the bladder the allantoic stalk 

 loses its cavity and becomes a solid rod, the urachus, leading 

 from the bladder to the umbilicus. The lumen usually disap- 

 pears early in the fifth week, but it may persist for a much 

 longer time, or even be present in the adult. 



3. The Mullerian Duct. 



About the end of the fourth week, a longitudinal ridge-like 

 thickening of the peritoneum appears along the outer side of each 



