402 E. V. COWT3RY 



early stages ; and the second that these mitochondria in the cells 

 of the neural tube apparenth' do not differ in any way, capable 

 of detection by the methods of technique now in use, from the 

 filamentous and granular mitochondria occurring in the struc- 

 tures derived from the other two germ layers (i.e., mesoderm and 

 endoderm) . 



MITOCHONDRIA AND NEUROFIBRILS IN LATER STAGES: EMBRYOS 



OF 15 TO 33 SOMITES 



1 . The stage of development at which the first neurofibrils are formed 



I find that neurofibrils are first formed in the nervous system 

 of a chick embryo, the differentiation of which may be charac- 

 terized as follows: Somites 15, length 5.8 mm., incubation at 

 39°C=fc 40 hours (being slightly more advanced than Duval's 

 embryo of 33 hours, fig. 268, p. 56). In such an embryo the neuro- 

 fibrils are most abundant in the marginal neuroblasts in the 

 hind-brain opposite the otic invagination (fig. 11). A few ceUs 

 containing them may be seen in the midbrain (fig. 12) ; but in the 

 forebrain the only indication of their presence is a more or less 

 continuous network, blackened with silver nitrate, in the distal 

 (remote from the membrana limitans interna) portions of the 

 cells (figs. 10 and 12). Traces of them occur further caudad in 

 the cells of the neural tube as far back as the fifth somite. There 

 is a large area comprising the posterior portion of the midbrain 

 and the anterior part of the hind-brain in which no neurofibrils are 

 differentiated, even in the stages as far advanced as 20 somites. 



Since the neurofibrillar methods of technique employed b}^ me 

 depend upon impregnation with silver nitrate, they are not suf- 

 ficiently accurate to permit of complete enumeration of the cells 

 in the different parts of the nervous system containing neurofi- 

 brils, and I have therefore been prevented from studying the 

 regional differentiation of neurofibrils on a percentage basis and 

 comparing my results with those obtained by Paton ('07) and 

 Coghill ('09, etc.) who worked on fish and amphibian embryos- 

 respectively. 



