CHAP, x.] THE DEVELOPMENT OF THE GAT. 351 



While the Wolffian "bodies are forming, another, single, invagina- 

 tion is formed on its inner side, and this closes over, extends, and 

 becomes a tube called the MULLERIAN DUCT. The two Mullerian 

 ducts, which end blindly at their anterior end, extend backwards, 

 meet and coalesce where they open (side by side and between the 

 two Wolffian ducts) into the proximal part or root of the allantois. 



The ALLANTOIS itself, as has already been said, grows forth as a 

 membranous sac from the hinder end of the ventral aspect of the 

 primitive alimentary canal, and extends out between the somato- 

 pleure and splanchnopleure to the inner surface of the chorion in 

 the umbilical cord. 



As the visceral plates of the somatopleure close round the belly of 

 the embryo, they come finally to embrace the umbilical cord where 

 it quits the body. 



The part of the allantois which is within the abdominal cavity, 

 becomes differentiated into what are ultimately (1) the proximal part of 

 the urethra, (2) the bladder and (3) the urachus : the urachus being that 

 part of the allantois intervening between the bladder and the umbilicus, 

 or navel. The cavity of the urachus becomes narrower and narrower 

 till it closes, and so this part becomes transformed into a ligament, 

 with traces, perhaps, of its primitive cavity. The proximal part of 

 the allantois opens at first into the hinder end of the primitive 

 alimentary cavity, but by degrees a partition (the future perineum) 

 grows forwards horizontally, and divides the dorsal alimentary 

 chamber, or rectum, from the ventral urogenital sinus, into which 

 latter (the commencing proximal part of the urethra) the Wolffian 

 and Mullerian ducts open (Fig. 158, A). 



The KIDNEYS begin to appear as two oval dark-coloured bodies, 

 each placed behind and above the very much larger Wolffian 

 body of the same side. Nevertheless, each kidney is at first larger, 

 compared with the bulk of the whole body, than in the adult. 



The kidneys themselves arise subsequently to the development of 

 their ducts, the ureters. These latter seem each to be formed from 

 a dilatation of part of the Wolffian duct, which becomes constricted, 

 and so the ureter arises as a separated-off diverticulum of such duct. 

 This diverticulum, however, grows downwards and backwards, and 

 opens, not into the urogenital sinus, but into the bladder (Fig. 158, A, 

 b) which is separated from that sinus by a constriction. 



From the anterior end of each newly formed ureter diverticula 

 grow forth into the mesoblast, and these diverticula lengthen, become 

 greatly contorted, and end in true renal Malpighian corpuscles. It 

 may be, however, as some contend, that the various parts arise 

 separately in the formative tissue, each part at first being solid, but 

 subsequently acquiring a cavity, and all ultimately uniting together. 



The SUPRA-RENAL CAPSULES arise quite independently of the 

 kidneys. For a time they are much larger than the last named 

 organs. They appear to originate as a single mass in front of the 

 kidneys, subsequently becoming divided into two lateral organs. 



The TESTES appear after the Wolffian bodies, but before the 



