VERTEBRATES FROM A CRUSTACEAN-LIKE ANCESTOR. 391 



of the animal has been treated with osmic acid (1 per cent.). 

 The result of such treatment is most striking, as is seen in figs. 

 6, 11, PI. XXV, and figs. 12, 13, PI. XXVI. 



We see in all cases that the white matter of the brain stains 

 of a greyish colour, and that the cells and cell nuclei of the 

 grey matter stain also of a greyish colour, their contour being 

 much less well defined than with carmine or hsematoxylin pre- 

 parations. The tissue of the cellular layer is less dense 

 than that of the white substance, and, being diff'erent also in 

 character, it stands out most conspicuously from the darker 

 coloured white-fibred layer. Further — and this is what is so 

 striking — the whole of this cellular layer is sprinkled over with 

 intensely black fatty granules, Avhich in the younger stages are 

 seen to be more especially congregated in the neighbourhood 

 of the lumen, as represented in figs. 13a — 13e, PI. XXVI. 



These fat-granules are confined to the grey matter ; and if 

 a very few black dots are to be seen in the white matter, they 

 are invariably found to indicate the position of one of the 

 nuclei similar to those found in the grey matter. At a 

 later stage, when the Ammoccete is near the time of its 

 transformation, the lumen of the brain-cavity is found to be 

 marked out in the most extraordinary way by this layer of 

 black fat-granules, which now form a dense black lining to the 

 central canal, marking out not only that part of the cavity 

 which is patent, but also the line of closure, wherever the 

 original cavity has become closed by its walls coming into 

 contact. The general appearance is shown in fig. 11, PI. XXV, 

 which represents a section, through the fourth ventricle, of an 

 Ammoccete which was near the time of transformation. Exami- 

 nation with a higher power (fig. 12, PI. XXVI) shows that only 

 the cell layers near the lumen are crowded with this dense mass 

 of fat-globules, and that a very distinct limiting line of tissue 

 separates the layer of fat-globules from the deeper lying cells 

 of the grey matter. In among the radially arranged cells of 

 this deeper layer fat-globules are seen scattered about here 

 and there up to the edge of the white-fibred tissue of the 

 medulla. 



