512 



THE TISSUES. 



Fig. 342. 



by their growth in length and thickness. In the fifth month of 

 foetal life, all the larger and medium-sized vessels are formed with 

 their coats and tissues, and no vestige of the formative cells remains. 

 The tissues are, however, still incomplete, the muscular fibres being 

 short and delicate, and there being only layers of slightly coalescent 

 fusiform cells, instead of the elastic membranes subsequently to 

 appear. 



2. The capillaries are developed in an entirely different manner 

 from the preceding, since they are formed by the coalescence of single 



cells. These are first arranged in 

 a line, then the contiguous cell- 

 walls are absorbed, with their con- 

 tents, but not their nuclei, which 

 still appear, therefore, in the wall 

 of the capillary vessel. Delicate 

 pointed processes then project from 

 the walls of these little vessels, 

 which rapidly elongate, and coa- 

 lesce with similar processes of stel- 

 late cells dispersed in the surround- 

 ing tissue. The other processes of 

 these stellate cells also unite, and 

 thus a network of stellate cells, 

 continuous with the already formed 

 capillaries, is produced. The pass- 

 ages in this network are at first 

 very minute and irregular, and do 

 not admit the blood-corpuscles; 

 but they rapidly enlarge, and be- 

 c'ome true capillaries. (Fig. 342.) 

 New connections are also frequent- 

 ly formed between already pervious 

 capillaries, by the direct meeting of 

 prolongations from them, and by 

 the mutual connection of forma- 

 tive cells lodged in their interstices. 

 3. The capillary lymphatics exhi- 

 bit essentially the mode of develop- 

 ment just described; except that 

 anastomoses are rare in them, and 



Formation of capillaries in tail of tadpole. 

 a, a. Capillaries permeable to blood. 6, 6. Fat- 

 granules attached to the walls of the vessels, 

 and concealing the nuclei, c. Hollow pro- 

 longation of a capillary ending in a point, d. 

 A branching cell with nucleus and fat-gran- 

 ules, communicating by three branches with 

 capillaries already formed, e. Blood-corpus- 

 cles still containing granules of fat. 



