CHANGES IN THE ALLANTOIS. 



a somewhat pear-shaped body protruding from the pelvic portion of the 

 trunk, and soon becomes vesicular. Shortly after its formation there is ob- 

 served a narrowing of that portion of the vesicular body nearest the foetus, 

 (fig. 1 5, 3) while the more distant portion undergoes a rapid increase in 

 size, and gradually approaches the chorion, to which structure the numerous 

 vessels surrounding the allantois are thus conducted from the body of the 

 foetus. This object attained, the development of the allantois is completed, 

 and the subsequent changes which it undergoes appear to be retrograde 

 ones. The first of these changes which ensues is the production of 

 a kind of twist or constriction on that side of the vesicular part of 

 the allantois turned towards the chorion (fig. 15, 7); this is the com- 

 mencement of the formation of the urachus. The vesicular part con- 

 stitutes the rudiment of the urinary Fig 15 * 

 bladder, and maintains its connection 

 with the body of the embryo by 

 means of the narrowed tubular part 

 (3) already alluded to (and which 

 has been hitherto, but erroneously, 

 described as the urachus). This tu- 

 bular part, indeed, forms a direct 

 communication between the urinary 

 bladder and the primordial kidneys 

 or Wolffian bodies, by dividing into 

 two portions, each of which passes 

 directly into the corresponding Wolffian body, and thus the ureters are 

 formed.f As the embryo increases in size, and the Wolffian bodies gra- 

 dually disappear to be replaced by the kidneys, the ureters together with 

 the urinary bladder which by degrees loses its round or elliptical form 

 to be elongated in the direction corresponding to the length of the 

 umbilical cord are drawn into the cavity of the pelvis. When this has 

 occurred the bladder assumes a club-shaped form, the base of which repre- 

 sents the fundus of the organ, while the apex tapers upwards to pass into 

 the urachus, which for some little distance is therefore tubular. Langen- 

 beck cannot speak with certainty respecting the length of time required 

 for the bladder to be thus drawn into the body of the embryo ; but he 

 considers that by the twentieth week the process is always completed, fre- 

 quently even by the twelfth. For long after this period, however, the 



* Fig. 15. Human foetus with the umbilical cord, allantois, and a portion of the chorion. 

 After Langenbeck. The right hind extremity has been removed. 2. Allantois (or rudiment 

 of the urinary bladder). 3 and 4. Umbilical cord. 5. Ureters. 6. Ductus vitello-intes- 

 tinalis. 7. Fold or constriction of the allantois, indicating the first formation of the 

 urachus. 



t This division and the junction of the branches with the extremities of the Wolffian 

 bodies, has been observed also by Professor Budge in a five-weeks old human embryo. 

 (Mttller's Archiv. 1847, p. 9.) 



