406 E. V. COWDRY 



in the rounded neuroblasts, in apolar cells, and in the irregular 

 shaped cells within the lumen of the neural tube they are either 

 in the form of granules or of straight or slightly curved rods with 

 rounded ends; whereas, in the bipolar cells they are longer and 

 straighter. I have never observed any network formation or 

 fusion of mitochondria in fixed preparations or in specimens 

 stained with janus green intravitam, although I have searched 

 carefully for it in the region of the axone and axone hillock. It 

 should again be emphasized that the illustrations tend to mini- 

 mize the uniformit}'- in the morphology of the mitochondria on 

 account of the fact that their long axes are directed in all planes 

 relati\'e to the drawing and that in sectioning they have been 

 cut at all angles. 



The mitochondria show no variation in their morphology of 

 which abundant examples may not be seen in neighboring struc- 

 tures derived from mesoderm and endoderm. Neither do they 

 differ in shape from the mitochondria occurring in the neural 

 tube before the formation of any neurofibrils. 



The appearance of neurofibrils is fairly constant in strictly 

 homologous cells of these four types. They are apparently 

 continuous, the mitochondria discontinuous: they seem to form 

 a reticulum, whereas the mitochondria never lost their individ- 

 uahty; and finall}^ the neurofibrils are of very fine diameter and 

 irregular outlines, in spite of the fact that the tendency of im- 

 pregnation methods is by precipitation, and in other ways, to 

 add to the size of the structures demonstrated. 



b. Distribution. The mitochondria, like the neurofibrils, are 

 generally arranged parallel to the long axis of the cell, as in the 

 case of the neuroblasts, the bipolar and epithelial cells. Occa- 

 sionally the neurofibrils are arranged in a whorl-like concentric 

 fashion about the nucleus, but the mitochondria are not simi- 

 larly oriented. Both neurofibrils and mitochondria occur in all 

 parts of the cytoplasm. In some specimens, stained with iron 

 hematoxylin, the mitochondria gather more thicklj^ in the pe- 

 ripheral parts of the cell, although such an accumulation is not 

 apparent in cells stained with janus green intravitam (figs. 23 and 

 24). 



