328 THOMAS J. HELDT 



his studies Molisch concludes : Freezing causes a separation of 

 water; this water at numerous points forms ice crystals which 

 by molecular force continue to extract water from the substance 

 in question and by their consequent enlargement push aside the 

 substance, and in this way give rise to the network or reticulum 

 observed. Several of his figures, one of which is reproduced here 

 in figure 1, strikingly illustrate this point. 



The obser^'ations on various gums and other colloids all agree, 

 in their essentials, with those of Molisch: Ambronn ('91); on a 

 number of inorganic and organic colloids, at temperature reduc- 

 tions of -10°, -70°, and -180°C. by Bobertag, Feist, and 

 Fischer ('08) ; on starch-paste, various gums, and hemoglobin, at 

 temperatures as low as — 180°C., by Fischer ('11); and, on gela- 

 tin by Liesegang ('11). Likewise, except for some details, the 

 observations of the author are in accord with those of the inves- 

 tigators named. 



The observations on frozen neural tissue may be considered 

 under the following divisions: (a) fresh, frozen, unfixed, and 

 unstained smears; (b) frozen, fixed, and stained smears consid- 

 ered as a whole; (c) nerve cells of smears frozen at —5° to — 10°C. ; 

 (d) nerve cells of smears frozen at — 10°C. and lower. 



a. Fresh, frozen, unfixed, and unstained smears. A smear of 

 gray matter from the spinal cord of dog, frozen over the gas- 

 escape of a carbon dioxide tank or by making the smear on a 

 very cold slide, becomes of whitish opacity and at very low tem- 

 peratures develops clefts not unlike those observed in ice and 

 frozen egg-white, though in general it takes a much lower tem- 

 perature to develop such clefts in neural tissue. The appearance 

 presented under the microscope varies considerably according to 

 the thickness of the smear, the rapidity of the freezing, the degree 

 of temperature reduction, and undoubtedly too on the surface 

 tension and other intrinsic forces of the individual neural ele- 

 ments. In general, however, it may be said that small foam- 

 cells resembling those of aqueous ice, variously arranged, are as 

 a rule to be observed around the margin of the smear; the smear 

 itself appearing as a coarse network or reticulum with large and 

 small, irregidar or multiangular meshes. Within the meshes is 



