36 CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



minute fibrils, figure 46. The network of fibrils is concealed by being covered 

 with flattened connective-tissue corpuscles, which may be readily dissolved 

 in caustic potash, leaving the network bare. The network consists of white 

 fibers, the interstices of which are filled with lymph-corpuscles. The cement- 

 substance of adenoid tissue is very fluid. 



Neuroglia. This form of connective tissue found in the nervous system 

 is described on page 77. 



Development oj Fibrous Tissues. In the embryo the place of the fibrous 

 tissues is at first occupied by a mass of roundish cells, derived chiefly from 

 the mesoderm, but also from ectoderm and from entoderm. These develop 

 either into a network of branched cells or into groups of fusiform cells, 

 figure 43. 



The cells are embedded in a semifluid albuminous substance derived 

 probably from the cells themselves. Later this formed material is converted 

 into fibrils under the influence of the cells. The process gives rise to fibers 

 arranged in the one case in interlacing networks, areolar tissue, in the other 



FIG. 44. Blood- Vessels of Adipose Tissue. A, Minute flattened fat-lobule, in which the vessels 

 only are represented, o, The terminal artery; v, the primitive vein; b, the fat-vesicles of one 

 border of the lobule separately represented. X 100. B, Plan of the arrangement of the capillaries. 

 c, on the exterior of the vesicles; more highly magnified. (Todd and Bowman.) 



in parallel bundles, white fibrous tissue. In the mature forms of purely 

 fibrous tissue not only the remnants of the cell-substance, but even the nuclei, 

 may disappear. The embryonic tissue, from which elastic fibers are developed, 

 is composed of fusiform cells, and a structureless intercellular substance. 

 The fusiform cells dwindle in size and eventually disappear so completely 

 that in mature elastic tissue hardly a trace of them is to be found: mean- 

 while the elastic fibers steadily increase in size. 



