416 E. V. COWDRY 



known, in response to physiological demands incident at the time 

 of their differentiation. The darker staining of the perinuclear 

 cytoplasm with silver nitrate (figs. 10, 12 and 14) in early neuro- 

 fibrillar stages strongly indicates the possibility that the nucleus 

 either contributes substance or participates in some way in the 

 formation of neurofibrils. 



SUMMARY 



1 . The neurofibrils are first formed in developing chick embryos 

 as a differentiation of the ground substance (in the majority 

 of cases of the peripheral neuroblasts) at a stage of develop- 

 ment characterized by possessing 15 somites, being about 5.8 mm, 

 in length and having been incubated for forty hours at 39°C. ± 

 (p. 402). 



2. In the early phases of the development of the chick the 

 earliest neurofibrils are formed in three chief localities: (1) in the 

 hind-brain opposite the otic invaginations (stages of 15 somites 

 on) ; (2) in the nuclei and root fibers of the cranial nerves (from 

 stages of about 15 somites on); and (3) in a center on either 

 side of the extreme anterior end of the midbrain (stages of 18 

 somites on) (p. 403). 



3. There is no evidence that mitochondria are transformed into 

 neurofibrils. The facts are these: (1) that there is no decrease 

 in the amount of mitochondria in development parallel to the 

 increase in the neurofibrils; (2) that the mitochondria do not 

 show, either by a variation in their morphology, staining reac- 

 tions, or in any other fashion, capable of detection by our pres- 

 ent methods of technique, indications of being transformed into 

 material of different chemical composition; and (3) that during 

 the first part, at least (embryos of somites 15 to 32) of the period 

 of formation of neurofibrils, the mitochondria in the neural tube 

 retain the similarity in their morphology and staining reactions 

 to the mitochondria occurring in the structures derived from the 

 other two germ layers (i.e.,* mesoderm and endoderm) which 

 they possessed before any neurofibrils became differentiated 

 (pp. 405^08). 



