98 THE ESSENTIALS OF HISTOLOGY 



stance of the connective tissue, and that their walls are in close con- 

 nection with the cells and cell-spaces of that tissue. But, except in 

 the case of the serous membranes, there is no open communication 

 between the lymphatic vessels and the interstices (areolae) of the con- 

 nective tissue. 



Development of the blood-vessels and lymphatics. The blood- 

 vessels and lymphatics are developed in the connective tissue or in the 

 mesoblastic tissue which precedes it, the first vessels being formed in 

 the vascular area which surrounds the early embryo. Both kinds of 

 vessels are developed from cells (vaso-formative cells) which become 

 hollowed out by an accumulation of fluid in their protoplasm, and in 

 the case of developing blood-vessels coloured blood-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 mean- 

 time 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. 123). These presently become connected with 



FIG. 123. ISOLATED CAPILLARY NETWORK FORMED BY THE JUNCTION OF 

 SEVERAL HOLLOW ED-OUT CELLS, AND CONTAINING COLOURED BLOOD-COR- 

 PUSCLES IN A CLEAR FLUID. 



c, 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. 



previously formed vessels, which extend themselves by sending out 

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

 known whether the larger blood-vessels and lymphatics are developed 

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

 added, or whether they are formed as clefts in the mesoblastic tissue 

 which become bounded by flattened cells. 



The se ous membranes, which may conveniently be studied in con- 

 nection with the lymphatic system, 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 



