HISTOLOGICAL DEMONSTRATIONS. 105 



number of tadpoles be kept for some days in unchanged 

 water, a remarkable tendency to diapedesis manifests itself; 

 the white corpuscles can be found in all stages of emigra- 

 tion. The diapedesis is, however, a very slow process ; a 

 corpuscle may take from one to three hours to pass through 

 the vascular wall, thin though it be. 



When a small drop of ammonia is placed on the tail, a 

 great emigration of white corpuscles ensues after a time. 

 Red as well as white corpuscles often pass out. The emi- 

 gration of the former probably results from pressure, for 

 they have no contractile power. 



Study the method of injecting blood-vessels ( 333-343). 



LYMPHATICS. 



169. Stomata of Lymph Sac (H.) Examine a 

 silvered preparation of the septum cysterncz lymphatics 

 magncz of the frog prepared thus : 



Open the abdomen of the animal just killed, and remove the 

 viscera. The great lymph sac is placed on either side of the spinal 

 column, immediately behind the stomach. Wash away the blood with 

 distilled water, pour a half per cent silver nitrate solution over it, and 

 allow to remain for three or four minutes ; wash thoroughly in dis- 

 tilled water, and expose to the light either in distilled water or in 

 glycerine until a brownish colour appear. Excise a portion of the sac 

 and mount it in glycerine. 



The silvered outlines of the transparent epithelial plates 

 that form the wall of the septum will be readily recognised. 

 Alter the focus, and observe that there are two layers of 

 these, the cells of the one the peritoneal being more or 

 less elongated, those of the other the layer belonging to 

 the sac being more rounded in form. In both, the out- 

 lines of the cells are somewhat sinuous. The stomata, or 

 apertures leading from the peritoneal into the lymph sac 

 are bounded by a special layer of very finely granular poly- 

 hedral cells, that are readily stained throughout by the 

 silver. These cells in diseased states often proliferate, 

 hence they have been designated " germinating," (Kiting 



Clusters of cells of this germinal serous epithelium may 

 often be found on the mediastinal pleura of the dog, and 



