416 W. n. GASKELL. 



such cases the pigment is in the position of a foreign body, 

 and acts as a mild irritant to the surrounding connective- 

 tissue cells, with the result of increased proliferation and in- 

 creased fibrous-tissue formation. 



I conclude, then, that in the Ammocoetes the formation of 

 pigment which is so characteristic of this animal is due to the 

 large amount of spleen-like tissue in this animal in the spaces 

 of which passive destruction of blood-corpuscles occurs, with 

 the result of the deposition of pigment in the walls of these 

 spaces, and the diminution in the size of the spaces themselves. 

 Conversely I conclude that lines and irregular masses of pig- 

 ment in any locality indicate the existence of former spleen- 

 like tissue — in other words, are evidence of the position of 

 obliterated lacunar blood spaces. 



If we now revert to our original question — what is the 

 meaning of the pigment found so markedly in among the 

 degenerate cells surrounding the brain ? the answer would be : 

 These lines and collections of pigment are the remains of the 

 blood channels which supplied the cells of the old cephalic 

 liver with blood. With the loss of function of this organ these 

 blood channels have become filled more or less with pigment- 

 bearing corpuscles, thus obliterating the greater part of the 

 original vascular space, certain blood-vessels only being left in 

 the ti'ssue itself, as shown in fig. 2, PI. XXV {b. v.). The 

 remnant of the blood space surrounding this tissue is still to be 

 seen in the shape of the so-called venous sinus found both on 

 the dorsal and ventral sides of the brain. The dorsal sinus is 

 figured in figs. 9, 14, and is a large blood space which surrounds 

 the choroidal plexus, and separates the two lateral masses of 

 arachnoidal tissue from each other. The pigment in between 

 the folds of the choroidal plexus (figs. 9, 14), which Ahlborn 

 considers to represent included arachnoidal tissue, is, to my 

 mind, much more probably due to the passive destruction of the 

 blood-corpuscles of this sinus ; and the vessel described by him 

 in the extremity of the pigmented folds is the remnant of the 

 original blood space. This sinus itself is perhaps homologous 

 with the longitudinal sinus of the higher Vertebrates. 



