400 E, V. COWDRY 



distinct, and ascribes the difficulty experienced by investigators 

 in the demonstration of cell boundaries to inadequate technique. 

 I have already ('12 a, p. 486) mentioned the precautions neces- 

 sary in the fixation of the cytoplasmic components of adult nerve 

 cells and the same apply without reservation to the embryo. 



(3) The inevitable contraction, resulting from dehydration has 

 been reduced by decreasing the period of immersion in the grades 

 of alcohol to a few minutes only, and by the use of bergamot oil 

 for clearing, which renders the use of absolute alcohol unnec- 

 essary. 



(4) It is difficult to dissociate the effects of fixation and imbed- 

 ding. The shrinkage due to imbedding in paraffin and celloidin 

 was slight. I fail to see any very great or constant differences in 

 the appearance of tissues treated by these two methods. In mj^ 

 experience the paraffin method does not produce any more arte- 

 facts than the celloidin method. 



(5) Sectioning, if carefully carried out, does not produce se- 

 rious artefacts (Cowdry '12 b, p. 491). 



(6) The methods of impregnation and staining control each 

 other, for the neurofibrils appear to be essentially similar in spec- 

 imens prepared by Cajal's and Paton's technique, and the mito- 

 chondria in preparations made by the methods enumerated in 

 table 2. 



MITOCHONDRIA IN EARLY STAGES BEFORE THE DIFFERENTIATION 

 OF NEUROFIBRILS: EMBRYOS OF TO U SOMITES 



Both filamentous and granular mitochondria undoubtedly ex- 

 ist in the nerve cells of the chick in these early stages, but the 

 filamentous form is much more numerous. The illustrations (figs. 

 1 to 5) tend to minimize the uniformity in the morphology of the 

 mitochondria on account of the fact that the long axes of the 

 mitochondrial filaments are directed in all planes relative to the 

 drawing and that they are cut in pieces of varying size at all 

 angles during sectioning. Most of the mitochondria are of neces- 

 sity oriented parallel to the long axis of the cells in which they 

 occur. The true filamentous nature of the mitochondria is there- 

 fore obscured in sectioned tissues, particularly in sagittal series 



