136 



DE VELOPMENT. 



week, after which time they begin to decrease in size and have nearly disappeared 

 by the end of the third month. The upper part of the segmental body, the prone- 

 phros, also undergoes atrophy and disappears. In the male, the Wolffian duct 

 persists, and becomes converted into the vas deferens, the Mullerian duct under- 

 going atrophy, a vestige of it, however, remaining as the sinus prostaticus; 

 whereas, on the other hand, in the female, the Mullerian duct remains and 

 becomes converted into the whole length of the genital passages, while the Wolf- 

 fian duct almost entirely disappears and remains only as a vestige. Prior to this, 

 however, the Wolffian and Mullerian ducts (together with the ureter when formed) 

 open into the common urogenital sinus referred to above, and which on its part 

 communicates with the terminal portion of the intestinal cavity which is known 

 as the cloaca (Fig. 105). 



As the allantois expands into the urinary bladder this common cavity is 

 divided into two by a septum, and the urogenital sinus then communicates with 

 the anterior division and the rectum with the posterior. The Wolffian and Mul- 

 lerian ducts are soon connected by cellular substance into a single mass the 

 genital cord in which the Wolffian ducts lie side by side in front, and the ducts 

 of Muller behind, at first separate, but later on uniting with each other. 



It has been stated that the kidney (metanephros) is developed from the lower 

 part of the Wolffian duct. It commences as a tubular diverticulum from the 

 lower part of the segmental duct, close to the cloaca. It extends upward, and 

 becomes divided into a number of csecal tubules, which represent the commence- 

 ment of the several divisions of the pelvis of the kidney. These tubules are 

 prolonged into a solid mesoblastic blastema situated near the lower end of the 



mesonephros. The tubules then become con- 

 voluted, and masses of cells accumulate on 

 their exterior, so as to give to the organ an 

 appearance of lobulation. Between these 

 cells vessels are developed, and the vascular 

 glomeruli are gradually formed. The kid- 

 neys at first, therefore, consist of cortical 

 substance only, but later on the proximal 

 ends of the tubes become straight and ar- 

 ranged in bundles, and thus the pyramidal 

 structure is developed. The lobulation of 

 the kidney is perceptible for some time after 

 birth. 



The urinary bladder, as before stated, is 

 formed by a dilatation of the lower part of 

 the stalk of the allantois. At the end of the 

 second month this forms a spindle-shaped 

 cavity, the bladder, which communicates with 

 the lower part of the primitive intestine by a 

 short canal, the urogenital sinus, which be- 

 comes the first part of the urethra. The 

 upper part of the stalk of the allantois, which 

 is not dilated, forms the urachus ; this extends 

 up into the umbilical cord, and at an early 

 period of embryonic existence forms a tube 

 of communication with the allantois. It is 

 obliterated before the termination of foetal 

 life, but the cord formed by its obliteration is 

 perceptible throughout life, passing from the 

 upper part of the bladder to the umbilicus, and 

 it occasionally remains patent in the adult, 

 constituting a well-known malformation. 

 The suprarenal bodies are developed from two different sources. The medul- 



FIG. 105. Diagram of the primitive uro- 

 genital organs in the embryo previous to sex- 

 ual distinction. The parts are shown chiefly 

 in profile, but the Mullerian and Wolffian ducts 

 are seen from the front. 3. Ureter. 4. Urinary 

 bladder. 5. Urachus. ot. The mass of blas- 

 tema from which ovary or testicle is after- 

 ward formed. W. Left Wolffian body. x. 

 Part at the apex from which the coni vascu- 

 losi are afterward developed. w, w. Right 

 and left Wolffian ducts, m, m. Right and left 

 Mullerian ducts uniting together and with the 

 Wolffian ducts in gc, the genital cord. ug. 

 Sinus urogenitalis. i. Lower part of the intes 

 tine. 



the labia majora or scrotum are formed. 



