CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



sheaths of the blood vessels. It forms sheaths for muscles, nerves, glands, 

 and the internal organs, and, penetrating into their interior, supports and con- 

 nects the finest parts. 



Structure. To the naked eye it appears, when stretched out, as a fleecy, 

 white, and soft meshwork of fine fibrils, with here and there wider films join- 

 ing in it, the whole tissue being evidently elastic. The openness of the mesh- 

 work varies with the locality from which the specimen is taken. Under the 

 microscope it is found to be made up of fine white fibers, which interlace in a 

 most irregular manner, together with a variable number of elastic fibers. 

 On the addition of acetic acid, the white fibers swell up, and become gelatin- 

 ous in appearance; but as the elastic fibers resist the action of the acid, they 

 may still be seen arranged in various directions, sometimes appearing to pass 

 in a more or less circular or spiral manner round a small gelatinous mass of 

 changed white fiber. The cells of areolar tissues are connective-tissue 

 corpuscles. 



V 



-4f 







FIG. 41. FIG. 42. 



FIG. 41. Mucous Connective Tissue from the Umbilical Cord, a, Cells; b, fibrils. 



FIG. 42. Part of a Section of a Lymphatic Gland, from which the corpuscles have 

 been for the most part removed, showing the Adenoid Reticulum. (Klein and Noble 

 Smith.) 



Gelatinous Tissue. Gelatinous connective tissue forms the chief 

 part of the bodies of such marine animals as the jelly-fish. It is found in 

 many parts of the human embryo. It may be best seen in the " Whartonian 

 jelly" of the umbilical cord and in the enamel organs of developing teeth. 



Structure. It consists of cells, which in the jelly of the enamel organ 

 are stellate, embedded in a soft jelly-like intercellular substance which forms 

 the bulk of the tissue. 



Adenoid or Lymphoid Tissue. Distribution. This variety of tissue 

 makes up the stroma of the spleen and lymphatic glands, and is found also 



