VEETEBKATBS FROM A ORUSTAOEAN-LIKE ANCESTOR. 417 



On the ventral side, in the region of the medulla oblongata, 

 the obliteration of the original blood space by the deposition 

 of pigment has also left a blood sinus which may be divided 

 into twOj one on each side of the middle line, as shown in 

 fig. 14. These sinuses are perhaps homologous with the 

 cavernous sinuses of the higher Vertebrates. 



Such I imagine to be the history of the pigment in the 

 tissue around the brain ; and this view is confirmed by its 

 appearance in those Ammocoetes which have been kept over a 

 year in the laboratory. As already mentioned, the accumula- 

 tion of large irregularly shaped pigment masses in the branchial 

 regions, in close connection with the occurrence of a great 

 number of pigment-bearing corpuscles, is a most striking result 

 of such confinement. Precisely the same kind of increase of 

 pigment is apparent in the pigmented tissue around the brain, 

 as is seen in fig. 17, PI. XXVI. 



The length of the Ammocoetes from which fig. 17 is taken 

 was 28 mm. ; it had remained in the laboratory from the 

 end of September, 1888, until October, 1889, and I doubt 

 whether it had increased in size at all. The increase of pig- 

 ment in connection with the branchia was very marked, and 

 the pigmented tissue around the brain had undergone most 

 marked modifications. In fig. 17 I have drawn apart of one 

 of the sections through the epichordal region of the brain, 

 and the difi'erence between it and the appearance it would 

 have presented in a fresh-caught Ammocoetes of the same 

 size is most striking; instead of closely packed liver-cells 

 with lines of pigment in between, we see that the cells have 

 disappeared, leaving only irregular-shaped clumps and shreds 

 here and there, while the pigment lines have become large 

 irregular clumps on a coarse network of connective tissue. 

 Close against some of these masses of pigment are seen blood- 

 corpuscles {a) lying in the ventral sinus, which bear masses 

 of pigment round their nuclei ; and the pigment in some of 

 them is indistinguishable from projections of the pigment 

 masses themselves. 



It is, then, clear that an increase of the pigment between 



