2 CYTOLOGICAL TECHNIQUE 



not possibly serve as representatives of other kinds. Extremes of 

 this sort are the flagellate spermatozoon, with strongly marked 

 polarity and scarcely any ground cytoplasm, and the ovum, with 

 a most unusual spherical or ovoid form and nearly always with 

 masses of reserve food-material preventing a clear view of the 

 other cytoplasmic inclusions. Embryonic cells are more un- 

 specialized. Those of animals are usually epithelial arid mesen- 

 chymatous; and since the latter are derived from the former, a case 

 can be made for the selection of an embryonic epithelial cell as 

 typical of cells in general. In fact, however, such cells usually 

 contain yolk granules, and these are much more evident than the 

 cytoplasmic inclusions that occur regularly in cells of all kinds. 

 The primary meristematic cells of plants are more unspecialized 

 than the embryonic cells of animals, and might indeed have been 

 selected for our purpose. 



Animals do not maintain many reserve stores of unspecialized 

 cells, but rather produce new cells, when required, by the multi- 

 plication of those that have already become specialized for par- 

 ticular purposes. Spermatogonia and oogonia, however, do con- 

 stitute reserve stores of unspecialized cells, and an examination 

 of them will reveal what parts are necessary for the retention of 

 life itself, as opposed to the performance of special functions on 

 behalf of the organism as a whole. An ordinary spermatogonium 

 may be regarded as more or less typical of the unspecialized cells 

 of animals, and it will serve here as an approximation to the ideal 

 'typical cell'. We shall see later how its structure is altered arti- 

 ficially by many of the usual processes of microtechnique. 



To determine the structure of a cell, it is essential to examine 

 it first while it is still alive. It can be examined in a body-fluid or 

 in a saUne solution of appropriate osmotic pressure. The cell 

 must be examined in its unaltered state by phase-contrast micro- 

 scopy, and also by direct microscopy after the use of vital dyes. 

 Subsequently it is proper to try every method of microtechnique 

 that can reasonably be supposed to throw light on the living 

 structure. 



The first thing to notice is the shape of the cell. Spermatogonia 

 do not maintain a particular form by the secretion of a solid cell- 



