60 HISTOLOGY. 



is apparently somewhat variable, for Mall has found in the 

 spleen two varieties of reticulum, one more and the other less 

 capable of withstanding the action of acids and alkalies. On 

 boiling, reticulum yields a small quantity of gelatin and a resi- 

 due of reticulin. The latter is a compound related to elastin 

 and gelatin. The gelatin obtained in this way is derived prob- 

 ably from white fibrous tissue mixed with the reticulum, as it is 

 impossible to obtain the latter absolutely pure. 



The reticulum of a lymph gland is shown in Fig. 34. The 

 lymphoid cells in this specimen have been shaken out, leaving 

 only the framework of the gland. 



Reticulum is distributed widely throughout the body, and 

 has been demonstrated in many organs. In the liver it is 

 identical with what Oppel has described as " Gitterfasern." In 

 the lymph gland, spleen, adrenal, intestine, lung, the capsules 

 of many organs, the testis, and the thyroid, reticulum has been 

 observed. Bone, cartilage, and the entire nervous system con- 

 tain no reticulum. In the pancreas, thymus, and heart there 

 is probably very little. 



(/) Fat tissue may, as mentioned above, be present any- 

 where in loose connective tissue. It appears in groups of cells 

 of the kind spoken of as signet-ring cells. 



Whether in the development of fat specific cells are con- 

 cerned, is not definitely known. The majority of authors state 

 that it can be formed from any fixed connective-tissue cells. 

 Others claim that it is only plasma cells, or cells resembling 

 these that have the power of collecting fat droplets within their 

 protoplasm. 



The very beginning of fat tissue, the so-called primitive 

 organ of the fat lobule (v. K(illiker), or the fat germinal layer 

 (Toldt), appears in the form of grayish-red masses, which consist 

 in each case of round membraneless cells with clear protoplasm, 

 in which under certain conditions fat is formed. This process 

 begins with the appearance in the protoplasm of fine small 

 highly refractive globules of fat which flow together. By 

 means of certain reactions (perosmic acid, which turns the fat 

 black ; Sudan III., which stains it red, and cyanin, blue) we 



