David, The Medulla of Petromyzon . 93 



shortness of the processes may be looked upon as an embryonic 

 condition, characterized by the slight differentiation of the nerv- 

 ous elements, while the absence of cornua anterior and posterior 

 seems to be a consequence of the flatness of the medulla. 



The course of the fibres is very clearly followed. As is well 

 known, the spinal medulla is flat and thin. Short pieces, i cm. 

 in length, were placed in the hardening fluid for about a week,^ 

 into silver Nitrate two or three days, and then cleared in Ori- 

 ganum or Dianthus oil, until they were transparent. In this 

 way beautiful preparations were obtained and the deeply stained 

 axis cylinders could be well followed. The thicker ones near 

 the central canal (fibres of Miiller) have a straight course, the 

 finer ones near the periphery a more undulating one. (See 

 Fig. 2.) 



These fibres could be followed through several of the i cm. 

 long segments, which I placed in the solution. With certainty 

 I was able to trace only the Miiller's fibres to their ending in the 

 acusticus and trigeminus nuclei. Transverse sections (Fig. i) 

 show that the fibres of Miiller are imbedded in tubes of the glial 

 substance. 



In their whole course they give off collateral branches, 

 which end above and below in the grey and white substance, 

 they oft^n divide dichotomously and end in fine, irregular 

 branches, often bending back into the grey matter, (Fig. 2). 

 They may take their origin in the cells, as above mentioned, or 

 from the dichotomous branching of the posterior roots, which 

 divide in the form of a T or Y, one branch passing forward 

 and the other backward. 



I never saw one of these " root branches" end in a cell, 

 but as above stated, the cells were not always well stained and 

 it may be only a result of the deficiency of the method. 



One sees no special arrangement of the elements in the 

 grey matter as in higher vertebrates, the distribution of cells and 

 fibres is very simple and seems to represent an embryonic stage. 



I am very sorry to state that I was not able to determine 



IWith igood results we added to the three per cent, bichromate of 

 potash solution crystals of the same salt ; these dissolve slowly and con- 



entrate the fluid. 



