686 MANUAL OF PHYSIOLOGY. 



The canal of the fore gut at a certain point becomes laterally 

 constricted, its transverse section presenting an hour-glass shape, 

 consisting of an upper and a lower dilated portion, united by a 

 central constricted neck. The lower of these cavities becomes 

 subdivided by the outgrowth of the lateral portions and the up- 

 growth of a part of the lower wall which forms a central septum, 

 so that the fore gut comes to be composed of an upper undivided 

 tube, giving off two appendages. 



These appendages consist of hypoblastic tissue, and as they 

 grow into the surrounding mesoblast they divide and subdivide, 

 until at last they come to consist of very minute tubules, which 

 terminate in dilated extremities. The undivided canal forms the 

 permanent trachea, the appendages the main bronchi, whilst 

 their minute subdivisions are the bronchioles, which terminate 

 in the dilated alveoli. 



The hypoblast forms the delicate lining membrane of the air 

 passages, and the mesoblast gives rise to the supporting tissue 

 holding them together, as well as to the blood vessels, the 

 muscular, cartilaginous, and connective tissue of the bronchial 

 tubes. 



The pleurae surrounding the lungs are, like the other serous 

 membranes, also mesoblastic in their origin. 



GENITO-URINARY APPARATUS. 



In the interval between the protovertebrse and the cleavage of 

 the mesoblast into its somatopleural and splanchnopleural layers, 

 there appears a mass of cells, which arrange themselves into the 

 form of a ridge. This ridge, which lies beneath the epiblast, 

 becomes hollow, and thus a tube is produced, which is called the 

 Wolffian duct. 



From this tube diverticula arise, which extend into the sur- 

 rounding mesoblast ; they are tubular, and communicate with the 

 central duct, whence they arise. The processes become twisted, 

 and at their extremities the neighboring mesoblast undergoes 

 differentiation, and forms vascular capsules corresponding in 

 structure to the Malpighian corpuscles. This part of the Wolffian 

 duct, which has acquired a glandular structure, is the Wolffian 



