MESENCHYME CELLS OF TISSUE CULTURES 433 



not, however, a homogeneous substance. The cells are flattened 

 out against the under-surface of the coversUp and present a 

 much more favorable condition for the observation of their 

 internal structure than the thicker, more irregular shapes in the 

 embryo. Furthermore, the living cells flatten (each into a com- 

 paratively thin plate, while functioning and readjust their internal 

 architecture in such a manner as to exaggerate more or less their 

 structure. It is usually possible to distinguish a clear homoge- 

 neous ectoplasm and a slightly darker, less homogeneous endoplasm 

 which seems to have a very finely powdered texture. It is thus 

 shghtly less transparent than the ectoplasm. There is no indi- 

 cation of a cell membrane distinct from the ectoplasm. Each 

 cell contains a centriole or centrosome located close to one side 

 or one end of the nucleus. I am not certain that I have ever seen 

 the centriole in the living cell, so that the proof of its existence 

 in the form of a small granule in these living cells is dependent 

 upon the condition found in fixed material. We cannot be sure, 

 however, that the granule which we call the centriole in the fixed 

 cell is not a coagulation product. It, however, undoubtedly rep- 

 resents a differentiation of the cytoplasm in this region which is 

 much emphasized in degenerating cells. Its position can usually 

 be determined by the general configuration of the cell contents, 

 especially if there is a slight amount of degeneration with a conse- 

 quent accumulation of granules about it. From the behavior of 

 the centriole (and centrosphere) in cell division and cell degenera- 

 tion, it is generally believed that it constitutes an important 

 cell organ or that certain activities are in some way centered in 

 or about it. 



The mitochondria are variable bodies, usually long and thread- 

 Hke and often branched in the more normal cells. In the older 

 cultures and in dividing cells they tend to break up into rods and 

 granules, and in extreme degeneration they may round up into 

 vesicles of various sizes. In the more normal cells they do not 

 appear to have any definite arrangement, but in degenerating 

 cells they tend to accumulate about the centriole or centro- 

 sphere, more or less radially in one type or stage of degeneration 



