12 HISTOLOGY 



cells known have increased their available surface by invaginations, 

 which form channels that carry lymph, blood, and waste matter. They 

 thus avoid the penalty of their size at the expense of extreme specializa- 

 tion. Figure 5 shows such a cell with a vascular circulation. Other 

 channels are found inside some cells which are used for internal trans- 

 portation or for the ultimate gathering of fluid and solid secretions and 

 their removal to gland lumina. These have been called trophospongia 

 by Holmgren, their discoverer, and they may be seen in Figure 12 as 

 well as other figures. 



The shape of the cell varies more greatly than its size. Cells, where 

 free from distorting lateral pressure, are as nearly spherical as possible. 

 Their position and use often cause them to assume very extraordinary 

 shapes, such as flat disks, long rods, and extreme branching forms. 



Technic. The same remarks that followed the part on protoplasm 

 will apply here. To get an accurate conception of the organization of any 

 kind of a cell, it is necessary to not only prepare this cell by a number 

 of the best methods, but to also make many preparations by the same 

 method. These latter preparations will differ much among themselves 

 as to the form of the different organs and the staining powers of the 

 different cell-substances. 



Flemming's strong fixing fluid is perhaps the best fixative known in the 

 majority of cases. It requires the most care and skill in its use, but gives 

 the truest pictures of the materials prepared by its agency. Zenker's 

 fluid, chrom-aceto-formol, and picro-acetic are examples of some of the 

 best of the other fluids, and they should all be tried. . 



LITERATURE 



The subject is so extensive and the literature consequently so large that the student is 

 referred to the general text-books. Wilson, "The Cell," Schneider, " Lehrbuch der 

 Histologie," and Hertwig's "Cell" will give extensive reading. 



