CYTOPLASMIC CONSTITUENTS OF NERVE CELLS 391 



I am much indebted to Prof, R. R. Bensley and to Prof. C. 

 Judson Herrick for theii- kind advice and invaluable criticism 

 in connection with this work. 



LITERATURE 



In this connection we shall consider briefly the literature deal- 

 ing with the mitochondria in early stages of development, the 

 theory relating to the transformation of mitochondria into neu- 

 rofibrils, the regional differentiation of neurofibrils, and finally 

 certain general points of criticism which apply to the work upon 

 the development of the cytoplasmic constituents of the nerve 

 cells of the chick as a whole. 



I know of no observations dealing with mitochondria in the 

 ectodermal structures of chick embryos of less than fifteen hours 

 incubation (Meves '08) ; but since the early stages have not been 

 characterized by smy data except the length of the period of in- 

 cubation it is difficult to say how early observations have act- 

 ually been made. Rubaschkin ('10) has described mitochondria 

 in guinea pig embryos as early as the four cell stage. Moreover 

 it has been shown that the mitochondria are distributed approx- 

 imately equally on cell division to the two daughter cells, and 

 that they are transmitted from one generation to another through 

 the medium of the egg and in all probability of the sperm also. 

 On account of considerations such as these the presumption is 

 warranted that the mitochondria occurring in the nervous sys- 

 tem of chick embryos are derived from those of the parental sex 

 cells. 



It has already been mentioned that Meves ('07, p. 403) was 

 the first investigator to claim that mitochondria played a role 

 in the formation of neurofibrils. He states subsequently ('08, p. 

 838), in the description of one of his figures, which was drawn 

 from a preparation of the spinal cord of a chick embryo of three 

 days and nine hours incubation and which shows a chain-like 

 arrangement of the chondi-iokontes (mitochondria) in the cyto- 

 plasm of the neuroblasts, that he regards this illustration as evi- 

 dence that these -chains of chondriokontes produce the primitive 

 neurofibrils, for he believes them to be the same threads which 



