GLANDULAR CELLS OF THE FROG'S PANCREAS 365 



before the latter can exert any destructive influence. Since the 

 penetration of acetic acid is far greater than the other reagents, 

 it is conceivable that the mitochondria in the deeper portion may 

 easily be destroyed by acetic acid before the other reagents could 

 get there. Hence, the fixatives with limited acetic acid, such 

 as Meves' and Benda's, bring to view the mitochondria in deeper 

 parts than in the above case. Finally, the fixatives which con- 

 tain no acetic acid preserve mitochondria evenly throughout the 

 piece. In the superficial portion of the piece, however, both cyto- 

 plasm and mitochondria are always better preserved than else- 

 where, no matter what fixative we use, provided that the dura- 

 tion of fixing is appropriately chosen. But it is worthy of remark 

 that osmic acid or sublimate is essential in order to preserve the 

 mitochondria, even in the cells of the superficial layer, in the 

 fixatives containing acetic acid. 



In a word, acetic acid, when a small amount of it is employed 

 in connection with other reagents, favors the fixation of the cell 

 without destroying the mitochondrial substance. On the other 

 hand, it can hardly be said that the fixatives which contain no 

 acetic acid always preserve mitochondria in a good state, since 

 poorly penetrating reagents cannot rapidly act upon the mito- 

 chondria in the deeper parts. 



b. Osmic acid is the most excellent fixing reagent. But this 

 alone, or mixtures with a great quantity of it, seems to cause 

 the shortening as well as thickening of mitochondrial filaments, 

 especially toward the centre of the piece. 



c. Formalin, used singly, also well preserves mitochondria. 

 But it cannot be said that this fixation is adapted for all of the 

 mitochondria stains. Applied to material fixed in this way, the 

 Benda stain gives very poor results, while iron-haematoxylin has 

 a greater affinity for the mitochondria in the deeper portions of 

 the piece. On the contrary, formalin material is most adapted 

 for Altmann's acid fuchsin, so that, under favorable conditions, 

 the most beautiful and instructive preparations can be obtained. 

 The affinity of formalin material for iron-haematoxylin can be 

 increased by treatment with osmic acid or osmium-sublimate 

 mixture after the formalin. 



