2 8 PRACTICAL HISTOLOGY. 



small quantity of a one per cent, solution of osmic acid for five hours. Wash them thoroughly 

 in water, and mount one in Farrant's solution. Cover. 



EXAMINATION (L). Note the skin and the subcutaneous tissue, and, scattered through- 

 out the latter, various sized groups of black spots. These black spots are of "various sizes, 

 and are the fat-cells, which have been blackened by the osmic acid. Select a group and 

 examine. {Indicate tliis general arrangement in PI. V., Fig. 3.) 



(H). Notice the different sizes of the various vesicles composing each group. Some of 

 the vesicles appear to be nearly filled with a blackened globule of fat, whilst others contain 

 only one or two small black granules of fat, the rest of the cell being occupied with a finely 

 granular protoplasm. These cells are connective-tissue corpuscles in process of being trans- 

 formed into fat-cells through the degeneration of their protoplasm into fat. Developing fat- 

 cells may be well studied in the connective tissue of the infra-orbital region of a young rabbit. 



PIGMENT IN CONNECTIVE-TISSUE CORPUSCLES. ' 



Connective-tissue corpuscles containing pigment are seen in the skin of the frog or in its 

 mesentery, and, in fact, in nearly all its tissues. Preparations will be obtained from the 

 choroid coat of the eyeball of any animal (p. 107). 



MUCOUS TISSUE. 



Mucous or embryonal tissue has a close relation to adipose tissue, as in the embryo it 

 forms the subcutaneous tissue which ultimately becomes panniculus adiposus in the adult. 

 It is essentially an embryonal tissue, as in addition to the site mentioned above, it occurs only 

 in the umbilical cord (Wharton's jelly), and in the vitreous humour of the eye. Acetic acid 

 precipitates the mucin in it in the form of fine threads. It consists of fusiform and branching 

 cells, whose processes communicate in all directions with each other, so as to form a mesh- 

 work, whose meshes are filled with a clear transparent mucin-yielding ground substance. In 

 it also are flat endothelial cells similar to those found in connective tissue. It soon changes 

 its character as the embryo grows older ; white fibrous tissue appears in the matrix, so that 

 it comes partially to resemble connective tissue, and then to pass into adipose tissue. In the 

 umbilical cord this last change does not take place : it remains in the fcetus at full time as a 

 fibrous tissue containing a mucin-yielding fluid in its meshes. 



PREPARATION (a). Subcutaneous Mucous Tissue. Inject into the axilla or groin of as 

 young an embryo as it is possible to get a small quantity of a quarter per cent, solution of 

 osmic acid, so as to form a bulla. Snip out a small part of the cedematous tissue, stain it 

 with logwood or picrocarmine, and mount in glycerine. 



EXAMINATION (L and H). Observe the fusiform and also the branched cells anastomo- 

 sing with each other with their nuclei stained. Use a very small aperture of the diaphragm. 



(b) Umbilical Cord. Harden part of the umbilical cord of a four months' fcetus in Miiller's 

 fluid for a week, and make transverse sections. Stain a section with logwood, and mount 

 it in Farrant's solution. Another may be stained with methyl-aniline (p. xliv) and mounted 

 in a saturated solution of acetate of potash. 



EXAMINATION (L and H). Observe the three umbilical vessels (p. 124) surrounded by 



