392 W. H. GASKELL. 



A comparison of the osmic specimens with those stained by 

 hsematoxylin shows that the main portion of the fat-globules 

 are situated in the position of the clear space which is so con- 

 spicuous in the latter specimens ; in other words^ the epithelial 

 layer of the central nervous system^ which is, upon my theory, 

 formed by the columnar epithelium of the original stomach, 

 is clearly differentiated from the rest of the grey matter at a 

 certain stage in the development of the Ammocoetes by its cells 

 becoming distended with fat, so that a marked limiting line 

 between these cells and those of the grey matter is brought 

 prominently into view. 



This accumulation of fat is due, in my opinion, to fatty de- 

 generation of the old stomach walls, rather than to the absorp- 

 tion of fatty particles, such as yolk-granules, which, I under- 

 stand, have been found in connection Avith the central canal of 

 the nervous system in the embryos of various Vertebrates. I 

 have come to this conclusion from the consideration of the 

 following facts. In the first place, these globules which stain 

 so black with osmic acid are really of a fatty nature, because 

 they dissolve away when the slide is placed in turpentine over a 

 water-bath for some time. In the second place, they are due 

 to degenerative changes, and not to absorption of yolk-particles, 

 because they increase in number as the animal grows ; in the 

 youngest specimens which I possess — about 23 mm. long — the 

 globules of fat are sparse and scattered about in the neighbour- 

 hood of the lumen, as shown in figs. 13a — e, PI. XXVI ; while 

 in the nearly full-grown Ammocoetes osmic specimens show the 

 dense black line of fat at the base of the columnar epithelial 

 cells which is so conspicuous in figs. 6, 8, 11, PI. XXV. Also 

 I have made an osmic preparation of one of the Ammocoetes 

 (70 mm. long) which had been kept alive in the laboratory 

 over a year without having increased in size. In this specimen 

 the layer of fat-globules is not only present, but also the fatty 

 change is much more marked than in fresh-caught specimens 

 of the same size. 



Further, the changes which take place in the cells of the 

 lining epithelium as the animal grows into its adult form are of 



