SPIXAL MARROW OF THE LAMPREY. 399 



have brought you for comparison a section from the spi- 

 nal marrow of one of the lowest of the vertebrate, namely 

 the lamprey (petromyzon). In this animal the spinal 

 marrow forms a very small, flat band which has some- 

 what of a depression on the surface, and at first sight 

 looks like a real ligament. On making a transverse sec- 

 tion of it, it is found to contain the same parts that we 

 see in man, but all only in a rudimentary form. What 

 in ourselves we call grey matter, is also found there on 

 both sides in the shape of a flattened' oblong lobe which 

 contains a few scattered ganglion-cells, but only very 

 few, so that perhaps only four or five are met with on 

 each side of the transverse section. In the centre a cen- 

 tral canal can likewise be detected, and that too lined 

 with an epithelial layer similar to that which occurs in 

 man. Below and in front of it generally lie a number of 

 largish, round cavities, which correspond to unusually 

 large, non-medullated nerve-fibres (Fig. 93, a), which 

 were first seen by Joh. Miiller. Farther outwards lie a 

 few other thick fibres, but greatly exceeding these in num-> 

 ber a large quantity of very fine fibres which give the 

 transverse section a very diversified, regularly dotted ap- 

 pearance. Among the ganglion-cells three different kinds 

 can here also be distinguished. Towards the outside of 

 the grey matter lie many-rayed cells, larger anteriorly, 

 smaller and, more simple posteriorly. More internally 

 and posteriorly on the other hand we find larger, more 

 rounded, seemingly diclonous (bipolar) cells, comparable 

 to the sympathetic forms. These cells communicate 

 across the middle-line by means of real fibres, and be- 

 sides we find processes which run out from the spinal 

 marrow forwards and backwards and form the anterior 

 and posterior roots. This is the simplest plan we have, 

 displaying these relations ; it is the general type of the 

 anatomical structure of these parts. 



