118 THE ESSENTIALS OF HISTOLOGY. 



corpuscles may also be formed within these cells (see Development of 

 Blood-corpuscles, Lesson II.) The cells branch and unite with one 

 another to form a network, and their cavities extend into the branches. 

 In the meantime their nuclei multiply and become distributed along 

 the branches, cell-areas being subsequently marked out around them. 

 In this way intercommunicating vessels capillaries containing blood 

 or lymph are produced (fig. 144). These presently become connected 

 with previously formed vessels, which extend themselves by sending 

 out sprouts, at first solid, and afterwards hollowed-out. It is not pre- 

 cisely known whether the larger blood-vessels and lymphatics are 



FIG. 143. ISOLATED CAPILLARY NETWORK FORMED BY THE .JUNCTION OF SEVERAL 

 HOLLOWED-OUT CELLS, AND CONTAINING COLOURED BLOOD CORPUSCLES IN A 



CLEAR FLUID. 



<:, a hollow cell the cavity of which does not yet communicate with the network ; p, p, pointed 

 cell-processes, extending in different directions for union with neighbouring capillaries. 



developed at first as capillaries, the muscular and other tissues being 

 subsequently added, or whether they are formed as clefts in the meso- 

 blastic tissue which become bounded by flattened cells. 



SKROUS MEMBRANES. 



The serous membranes, which may be conveniently studied in con- 

 nection with the lymphatic sj^stem, are delicate membranes of connec- 

 tive tissue which surround and line the internal cavities of the body, 

 and are reflected over many of the thoracic and abdominal viscera : in 

 passing to which they form folds, within which blood-vessels, lymphatics, 

 and nerves pass to the viscera. 



The inner surface is lined by a continuous layer of pai'din-nl- 

 epithelium (serous endothdium) (fig. 144), which is very distinct in nitrate 

 of silver preparations. In some places there are apertures in the 

 epithelium which lead directly into subjacent lymphatic vessels. These 

 apertures are called stomata, and are surrounded by small protoplasmic 

 cells (fig. 145 s, s). They are most numerous upon the peritoneal sur- 



