NORMAL HISTOLOGY. 



FIG. 30. 



and especially the septum separating the peritoneal sac from the 

 abdominal lymph-cavity of the frog, exhibit well these pores. The 



larger stomata are lined by 

 several small granular guard- 

 cells, whose expansion and 

 contraction largely influence 

 the size of the openings. 



The development of 

 epithelium is intimately 

 associated with the exten- 

 sions of the great ecto- and 

 entodermic tracts, since, with 

 the exception of the epithe- 

 lium of the greater part of 

 the genito-urinary organs, 

 the epithelia are the direct 

 descendants of the outer and 

 inner embryonic layers. The 

 cells lining the passages con- 

 nected with the sexual glands, 



as well as the urinary tract as far as the bladder, are derived from 

 those of the Wolffian body and duct, and hence have, with these 

 latter, a common mesoblastic origin. The simple arrangement of 

 the cells in the earlier stages gradually gives place to the more com- 

 plex disposition of the mature tissue. 



The development of endothelium forms part of the history of 

 the changes taking place within the extensive mesodermic areas ; 

 from the specialized sheet, or mesothelium, bounding the primary 

 body-cavity of the young embryo, the endothelium of the pleural, 

 pericardial, and peritoneal cavities directly descends, while the lining 

 cells of the vascular and lymphatic channels trace their origin to the 

 differentiation of certain of the mesodermic elements. 



Endothelium from the septum cisternae of frog, stained 

 with silver: a, one of the true stomata, lined with 

 guard-cells ; b, intercellular cleft ; n, nucleus. 



