DEVELOPMENT OF THE ADIPOSE TISSUE. 985 



Elastic Tissue. 



In the development of this tissue, which is nearly always associated or in- 

 termixed with the connective, some of the cells of the embryonal tissue pass 

 through the changes just described as proper to connective tissue cells, whilst 

 others subsequently become much elongated and attenuated, branch out 

 into fine ramifications, which unite with those of adjacent cells, undergoing 

 similar changes, and preserve their nuclei in their central or thickest part. 

 Sometimes these nuclei are ramified, at other times they are fusiform. In 

 many situations, as in fibrous membranes, ligaments, and tendons, in the 

 true skin, in the cornea of the eye, and elsewhere, these ramified and united 

 cells remain in the above-described condition, their nuclei forming the con- 

 nective tissue corpuscles. It has been said that they are hollow, and that 

 their fine ramifications convey a nutrient plasma through the dense fibrous 

 tissues in which they lie, and hence these have also been called plasm-cells. 

 Lastly, they have been said to form the commencements of the lymphatics ; 

 but this is not proved. According to some, the intercellular substance be- 

 comes fibrillated, and appears to be a sort of deposit from, or an excretion 

 upon, the nuclei or cells. 



In other situations, as in the loose areolar connective tissue, these fine fibres 

 appear darker, and form networks of elastic fibres. These reticular fibres, 

 becoming thickened by a deposit on their outer surface, form the stronger va- 

 rieties of elastic tissue ; by being joined or further thickened in parallel planes, 

 they give rise to the elastic fibrous networks of the air-tubes, or to flat fenes- 

 trated elastic membranes, as in the arteries. The true elastic fibres are said to 

 lose their nuclei ; but they may be merely covered in and hidden. 



Adipose Tissue. 



The vesicles of this tissue, however small or large, are obviously nucleated 

 cells, in which fatty matter has accumulated ; their nuclei become obscured, 

 or even disappear. The fat-cells have been traced in all stages of develop- 

 ment, from minute cells lying in the embryonal connective tissue, at first con- 

 taining only an albuminous fluid, then a few scattered oil-drops, and finally 

 attaining their fully distended condition. Other cells adjacent to them become 

 changed into areolar connective tissue, bloodvessels, or lymphatics, which 

 hold the fat-cells together, and minister to their nutrition. Often, when the 

 fatty matter is absorbed from the cells, in emaciation, the nuclei again become 

 visible. In the marrow of bones, the nucleated cell structure is very clearly 

 and beautifully seen. The reddish parts of the marrow also contain smaller 

 cells, and often compound nuclear cells, not fatty, called the medullary cells ; 

 these are very numerous in growing bone. 



Cartilage, Fibro- Cartilage, and Yellow Cartilage. 



The development of cartilage is more easily traceable than that of any other 

 tissue, from nucleated cells which are the immediate descendants of the primi- 

 tive embryonal cells. In the simplest form of cartilage, as in the chorda dor- 

 salis, and in certain other embryonal cartilages, the cells have very delicate 

 walls, which are closely applied together, grow into a polyhedral shape, and 

 present at first no appreciable intermediate matrix. More commonly, as in 

 the cartilaginous pieces which precede the formation of the bones, and in ar- 

 ticular cartilage, the original delicate cell-walls acquire an outer deposit or 

 secondary cell-wall, which goes on thickening, blends with that of other cells, 

 and also with a surrounding intermediate matrix. This is partly formed by 

 the secondary membranes, but partly, it is said, in the blastema between the 

 cells. This matrix either remains transparent, or becomes granular, or even 

 more or less fibrillated. The cells themselves enlarge, and new ones arise 

 within them, by successive endogenous binary subdivisions of the nucleus, ac- 

 companied by corresponding constrictions of the cell itself. Moreover, sec- 



