DEVELOPMENT. 201 



or anus may also be imperforate. This is caused by the non-union 

 of the segments developed from the epiblast with those developed 

 from the hypoblast. 



How are the glands of the alimentary tract developed ? 



(1) The salivary glands are developed from the epiblast lining the 

 oral cavity. They appear primarily as a simple tube which devel- 

 ops branches, about which the alveoli are formed. 



(2) The pancreas is similarly developed from the hypoblast of 

 the fore-gut. 



(3) The liver is primarily a protrusion of the hypoblast of the 

 fore-gut, which appears as soon as the blood-vessels begin to show 

 themselves. The omphalo-mesenteric vein, from the umbilical 

 vesicle, breaks up into a capillary plexus, and the hepatic cells 

 develop about it. 



How are the lungs derived? 



They first appear as a bud at the junction of pharynx and 

 oasophagus which soon forms a separate tube (the trachea). The 

 cells from the hypoblast extend into the surrounding mesoblast, 

 and it is from this structure that all of the tissue of the lungs, 

 except its mucous membrane, are formed. 



What is the Wolffian body? 



It is a rounded body which is first seen as early as the third 

 week as an increase of the cells of the mesoblast, just inside of 

 its division into parietal and visceral layers, on each side of the 

 vertebral column. It is soon seen to consist of three parts, from 

 which are derived the genito-urinary organs. The largest of these 

 in early embryonic life retains the name Wolffian body, and is not 

 a permanent organ ; a second, lying just above, develops the internal 

 organs of generation ; while the third, lying behind the Wolffian 

 body, is the rudimentary kidney. 



What is the function of the Wolffian body? 



The Wolffian body proper that is, after its division into three 

 sections is a temporary kidney. At first this is a large glandular 

 body, resembling the kidney in structure, which possesses a duct 

 leading to and opening into the hind-gut. At about the sixth 

 week of foetal life the kidney begins to grow and the temporary 

 organ to atrophy. As this occurs a duct for the kidney (the 

 ureter) is developed from the Wolffian duct. The use of the 



