VERTEBRATES PROM A CRUSTACEAN- LIKE ANCESTOR. 393 



such a kind as to confirm the view that we are dealing here 

 with true degenerative changes. We find, in the first place, 

 evidence that the formation of the substantia centralis 

 gelatinosa is partly due to the transformation of the cylin- 

 drical cells which line the central cavities. In the second 

 place, we find that, in those parts where the lining surfaces come 

 into contact, a process of stitching together of the two sur- 

 faces takes place, so as to form a seam, or raphe, as it is called 

 by anatomists. This fusion of the two layers of cylindrical 

 cells so as to form a seam is largely due to the fatty degenera- 

 tion and breaking down of the cells. 



(1) The Formation of the Substantia Centralis 

 Gelatinosa. — In the adult Petromyzon, Ahlborn describes 

 (3, p. 247) the changes which take place in the grey substance 

 as we pass from the spinal cord to the medulla oblongata ^' as 

 consisting in part of an extraordinary increase in the so- 

 called connective tissue or non-nervous elements, so that 

 these, embedded in a thick tangle of the finest fibres, now 

 form a broad zone around the lining epithelium and spread 

 laterally in between the ganglionic elements of the grey sub- 

 stance. They form the greater and compact central part of 

 the grey substance, on the outside of which the ganglion-cells 

 are imposed as a peripheral cortex.^^ 



In the adult condition, then, we find that the cellular 

 portion of the grey substance is separated from the lining 

 epithelial layer by a space which is composed of a complicated 

 mass of the finest fibrils, some of which form supporting 

 structures, others are probably nervous. The appearance of 

 a basement membrane to the columnar epithelial cells has 

 disappeared, and these latter cells themselves have altered 

 very much in appearance ; they can no longer be called 

 columnar epithelium, for, as Rohon (6) has described and 

 figured, and as is seen in fig. 10, PI. XXV, the basal part of 

 the cell is drawn into a long-tailed process, which is lost in 

 the fibrillar tissue of the substantia centralis gelatinosa; 

 the protoplasm of the cell is very scanty, so that the nucleus 

 fills up the greater part of the body of the cell, and the nuclei 



