THE WOLFFIAN DUCT. 689 



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 bloodvessels, the muscu- 

 lar, cartilaginous, and connective tissue of the bronchial tubes. 



The pleurae surrounding the lungs are like the other serous 

 membranes, also inesoblastic in their origin. 



GENITOURINARY 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 



FIG. 276. 



Transverse section through the embryo of a chick on the second day where 

 the medullary canal is closed. (Kolliker.) mr. Medullary canal. A. Epi- 

 blast. uwh. Cavity of protovertebra uw. ung. Wolffian duct. mp. Meso- 

 blast dividing into hpl. Somatopleure. df. Splanchnopleure. sp. Pleuro- 

 peritoneal cavity, dd. Hypoblast. ch. Notochord. 



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 dif- 

 ferentiation, and forms vascular capsules corresponding in struc- 

 ture to the Malpighian corpuscles. This part of the Wolffian 



58 



