The Allantois 353 



the mother. It is obvious that the meconium observed in the 

 amniotic fluid of the foal has been expelled from the intestine 

 through the anus and that, the anterior end of the alimentary tract 

 opening also into this cavity, any discharge of secretions or 

 debris from the mouth or nostrils falls at once into the amniotic 

 cavity. It is also clear that urine may be readily and freely ex- 

 pelled into this cavity and that the urinary salts found in the 

 amniotic fluid are derived from the fetal kidneys and discharged 

 into the amniotic s^c through the urethra. 



The amnion consists, on the embryonic side, of a thin layer of 

 epiblast, while, on the outer side, it is formed from the somato- 

 pleuric mesoblast. The external surface of the amnion is in 

 contact throughout more or less of its extent, varying according 

 to species, with the inner wall of the allantois. 



The Allantois. 



The allantois arises as an evagination from the hind gut just 

 posterior to the vitelline duct and grows outward and backward 

 between the two amniotic layers, and, in the mare and carnivora, 

 finally envelops the amniotic sac completely, so that, in these 

 animals, there occur two complete envelopes to the fetus, 

 each of which is eventually filled with fluid. (See Figs. 74, 75.) 



Tracing the allantois from its origin in the hind gut, there is 

 a slight constriction which is to constitute the urethra, followed 

 by a dilation from which the urinary bladder is to develop, then 

 an elongated, narrow tube, the urachus, which extends from the 

 fundus of the urinary bladder out through the umbilicus and 

 along in the umbilical cord across the amniotic cavity. 



In structure, the inner layer of the allantois is of hypoblast 

 derived from the inner layer of the hind gut, while the external 

 layer consists of mesoblast, in which the allantoic vessels are 

 formed to constitute the vascular layer. 



In all our domestic animals, the allantois constitutes a great 

 sac, which is filled with a liquid, the allantoic fluid, which con- 

 sists largely of the secretions from the kidneys, augmented per: 

 haps by transudation from the blood vessels of the walls of 

 the sac itself. The allantoic fluid contains albumin, grape sugar 



23 



