382 



HISTOLOGY 



stained red in contrast with surrounding blue fibrils. Within the body of 

 the embryo the allantois is prolonged to the upper end of the bladder, 

 with which it is continuous; this intra-abdominal part has long been 

 called the urachus (i.e., vas urinarium). If it remains pervious at birth, 

 which is abnormal, urine may escape at the umbilicus. 



The yolk stalk, surrounded by an extension of the body cavity, is 

 found in young umbilical cords (Fig. 382, A). This stalk is a slender strand 

 of mesoderm, containing the entodermal vitelline duct, and the vitelline 

 vessels which accompany it to the yolk-sac. The loop of intestine 

 from which the yolk stalk springs may also extend into the cavity of the 

 cord, and if it has not been drawn into the abdomen at birth, umbilical 

 hernia results. If the cavity of the vitelline duct remains pervious at 

 birth, the intestinal contents may escape at the umbilicus. (Such a con- 

 dition is known as a fecal fistula, whereas the pervious urachus constitutes 

 a urinary fistula.) Ordinarily the yolk stalk and its vitelline vessels, to- 

 gether with the ccelom of the cord, have been obliterated before birth, so 

 that no trace of them remains in sections of the cord. 





FIG. 384. YOLK-SAC A NO PERSISTENT 

 VITELLINE VESSELS, EXPOSED BY RE- 

 FLECTING THE AMNION AT THE DISTAL 

 END OF THE CORD. (Lonnberg.) 



FIG. 385. PART OF A HUMAN AMNIOTIC 



VlLLUS. X 330. 



Ep., Epitrichium; S. C., stratum corneum; 

 S. g., stratum granulosum; S. G., 

 stratum germinativum; M. B., homo- 

 geneous layer; F. T., fibrous tissue; 

 . T., areolar tissue. 



ge 

 A. 



The yolk-sac may be found with almost every placenta, as a very small 

 cyst adherent to the amnion in the placental area. If the distal end of 

 the cord is gently stretched, a wing-like fold appears (Fig.. 384), differing 

 from all others by containing no large vessels; the fold indicates the direc- 

 tion of the yolk-sac, which may be exposed by stripping the amnion from 

 the chorion. It may be beyond the limits of the placenta. Further 

 details will be found in Lonnberg's admirable Studien tiber das Nabel- 

 blaschen, Stockholm, 1901. 



Amniotic villi are irregular, flat, opaque spots on the amnion near the 

 Distal end of the cord. They are often present and may suggest a diseased 



