FAETIIEE DEVELOPMEXT OF THE URINARY ORGAXS. 813 



the hilus become straighter. and thus fomi the medullary substance. The tubuli, 

 as sho-wTi by Valentin, are absolutely, as 'u-ell as relatively, wider in the early 

 stages of formation of the kidney. The Malpighian corpuscles have been 

 seen by Rathke in a sheep's emin-yo, the kidneys of which measured only two 

 and a half lines in length. Koliiker observed the kidneys already foi-med in tlie 

 human embryo of between six and seven weeks, the ureter being hollow, and 

 communicating with dilated cavities within the rest of the blastema. At eight 

 weeks they had assumed their characteristic reniform shape, and about the tenth 

 week they are distinctly lobulated. The separate lobules, generally about fifteen 

 in number, gi-adiially coalesce in the manner already described ; but at birth, 

 indications of the original lobulated condition of the kidney are still visible on 

 the surface, and the entire organ is more globular in its general figui-e than in 

 the adult. The kidneys are then also situated lower do-mi than in after-life. 



In the advanced fcetus and in the new-born infant, the kidneys are relatively 

 larger than in the adult, the weight of both glands, compared with that of the 

 body, being, according to Meckel, about one to eighty at bu-th. 



The Suprarenal Boclies. — These organs have their origin in a mass of 

 blastema, placed in front of and between the kidneys and the Wolffian bodies. 

 They appear to originate in a single mass, and afterwards to become divided. 

 Koliiker has also observed them in close connection with the substance in which 

 the large sympathetic plexus of the abdomen is produced, but it is not ascertained 

 that they have a common origin. 



In the human emlnyo the suprarenal bodies are at the seventh or eighth week 

 larger than the kidneys, and quite conceal them, but after that time their relative 

 size diminishes, so that at about the tenth or twelfth week they are smaller than 

 the kidneys. At six months, according to Meckel, the proportion of the supra- 

 renal bodies to the kidneys is as 2 to 5 ; at birth the proportion between them is 

 1 to 3, whilst in the adult it is about 1 to 22. They diminish much in aged per- 

 sons, and are sometimes scarcely to be recognised. 



The Urinary Bladder and Urachus. — It lias elsewhere been stated 

 that in the human eniljiTO the vesicular part of the allantois extending 

 beyond the umbilicus is closed at a very early period. Its pedicle, how- 

 ever, remains in communication with the urogenital sinus, and receives 

 the ureters from the developing kidneys. The lower part of the pe'dicle 

 undergoes a gradual dilatation to form the urinary hladder, while at the 

 connection of this part with the urogenital sinus a constriction occurs 

 in the part which gives rise to the urethra. Tliis in the female opens 

 at once into the original urogenital sinus, but in the male the passage 

 is continued onwards through the penis by the median union of the 

 parts below that organ. 



The part of the allantois situated above or in front of the bladder within 

 the abdomen remains very much narrowed as the urachus, a tapering 

 process of the upper extremity of the bladder into which at first the 

 internal cavity is prolonged, but which later consists only of the 

 muscular and fibrous coats. This process may for a time be traced for 

 a short distance within the umbilical cord, but at an early period all 

 vestiges of its farther prolongation disappear. 



Genital Cord; — In both sexes, as was first fully shown by Tiersch 

 and Leuckart in 1852, the two Wolffian ducts become united by sur- 

 rounding substances into one cord behind the lower part of the urinary 

 bladder ; but retaining internally their separate passages until they 

 reach the sinus urogenitalis. With this cord the Miillerian ducts are 

 incorporated posteriorly, so that at one time there are four passages 

 through the whole of the genital cord. The Miillerian ducts next 

 coalesce into one at some little distance from their lower ends, and this 

 fusion, progressing upwards and downwards for a considerable space, a 



