PLATE 4 



EXPLAXATION OF FIGURES 



This plate logically belongs to the preceding (plate 3) in connection with wliich 

 it has been described. All the figures are from preparations made by Cajal's 

 silver nitrate method. 



15 b A portion of the neural tube from opposite the sixth somite on the right 

 side of an embryo of 17 somites, 44 hours and 15 minutes incubation. No neuro- 

 fibrils are formed, but the cytoplasm in the distal portions of the cells (a), (b) 

 and (c) is stained especially darkly with the silver nitrate. 



16 b Same region, embryo of 20 somites, length 7.4 mm., 88 hours, and 20 

 minutes incubation. The neurofibrils have appeared, in a single cell, and are 

 distinctly stained. 



17 b Same region, embryo of 22 somites. Considerable increase in neurofi- 

 brillar material. 



18 b Same region, embryo of 24 somites, length 7.5 mm., 65 hours incubation. 

 Shows some neurofibrils passing through the membrana limitans externa into the 

 myoblast. 



19 b Same region, embryo of 28 somites, length 6.8 mm., 63 hours incubation. 

 Neurofibrillar material increased. 



20 b Same region, embryo of 31 somites, length 7.3 mm., 62 hours incubation. 

 Shows a great increase in the number of neurofibrils, many of which now run par- 

 allel to the long axis of the neural tube. 



PLATE 5 



EXPLANATION OF FIGURES 



Figures 21, 26 and 25 represent progressive stages of differentiation in Bens- 

 ley's anilin fuchsin toluidin blue method. These figures have been drawn from 

 neighboring sections of the same embryo, mounted on the same slide. In the 

 first (fig. 21) the differentiation is practically nil, the mitochondria staining ex- 

 actly the same color as the neurofibrils; in the second (fig. 26) it has been carried 

 a little further with the result that the neurofibrils have lost their bright crimson 

 color and have assumed a dull red shade; while in the last (fig. 25) the decolori- 

 zation has been carried to an extreme so that the neurofibrils have lost all of the 

 acid fuchsin and have become stained with the differentiator, toluidin blue. It 

 is to be noted that in these progressive stages of differentiation the initial affinity 

 of the neurofibrils for an acid dye (acid fuchsin), in which they resembled mito- 

 chondria, is gradually changed to an affinity for a basic dye (toluidin blue), while 

 the intensity of the coloration of the mitochondria remains unaltered. Con- 

 clusions based upon the apparent similarity in the staining reactions of mito- 

 chondria and the primitive neurofibrils should therefore be received with caution 

 (p. 410 and 416). 



21 Cell and accumulation of neurofibrils from the ventral portion of the neu- 

 ral tube of an embryo, length from cervical flexure to tail flexure 8.4 mm., 100 

 hours incubation, corresponding closely in degree of differentiation to Duval's 

 embryo of 96 hours. Stained by Bensley's anilin fuchsin toluidin blue method 



426 



