CYTOPLASMIC CONSTITUENTS OF NERVE CELLS 403 



In general, the earliest neurofibrils appear to be formed in 

 chick embryos in three chief localities: (1) in the hind-brain oppo- 

 site the otic in^-agination (stages of 15 somites on) ; (2) in a center 

 on either side of the extreme anterior end of the midbrain 

 (stages of 18 somites and on) ; and (3) in the nuclei and root 

 fibers of the cranial nerves (from stages of about 15 somites on). 

 They are not differentiated until very much later (i.e., beyond 

 31 somites stages) in spinal ganglion cells. 



2. Type of cell in which neurofibrils first develop 



I hsLve found neurofibrils in bipolar cells, in apolar cells, in 

 marginal neuroblasts and in certain cells which occur within the 

 ca\uty of the neural tube. 



The neurofibrils in a typical bipolar cell are illustrated in fig- 

 ure 7. The cell is located on the right side of the hind-brain op- 

 posite the otic invagination of an embryo of 24 somites, length 

 7.5 mm., and sixty-five hours incubation, prepared by Cajal's 

 silver nitrate method. The cytoplasm is stained a little bit 

 darker yellow in the distal portion of the cell and the neurofibrils 

 appear thicker and more continuous than in the proximal part. 



The neurofibrils in apolar cells bordering the central canal 

 are best shown in figure 22. This specimen was prepared by 

 Paton's modification of the Bielschowsky silver impregnation 

 method. The cells illustrated occur in the neural tube opposite 

 the posterior part of the third somite of an embryo of 33 so- 

 mites, length 8.3 mm., and seventy-four hours incubation. At a a 

 continuous circumnuclear neurofibrillar network is seen in black 

 against a dull red granular background. The appearance of such 

 a neurofibrillar network in apolar cells is of distinctly rare oc- 

 currence. No networks of this nature were observed in embryos 

 of 15 somites, which is the stage of development at which the 

 neurofibrils first appear. 



Marginal neuroblasts containing neurofibrils niSLy be readily 

 found in embryos of all stages intermediate between 15 and 32 

 somites, and they probably occur in further developed embryos 

 also. Figure 11 shows the primitive neurofibrils in a marginal 



