VERTEBRATES FROM A CRUSTAOKAN-LIKE ANCESTOR. 407 



surface of the brain, and I will now briefly point out how the 

 examination of sections still more clearly defines the limits of 

 this tissue. 



A series of sections, both transverse and horizontal, through 

 the whole brain show that it forms two lateral masses which 

 do not spread dorsally beyond the choroid plexuses, so that 

 the latter, with the blood space surrounding them, are pressed 

 against the roof of the cranial cavity ; anteriorly at the 

 termination of the choroid plexus ii it reaches to the dorsal 

 roof, and fills the spaces round the pineal eye and ganglion 

 h-abenulae, as seen in figs. 20a — 20c?, PI. XXVII, spreading 

 over and in front of the cerebral lobes, even to the space between 

 the two olfactory nerves, as in fig. 6, Pl.XXV. On the ventral 

 side it does not exist in the infundibular region, for here the 

 pituitary body and the infundibular projection come into close 

 contact with the floor of the cranial cavity. In the region of 

 the medulla oblongata it is at first very scanty on the ventral 

 side, but thickens as the termination of the conus post-commis- 

 suralis is reached, until at last in the region of the large ventral 

 commissure it attains a considerable thickness, so that here, 

 and here only on the ventral side is there anything in the 

 nature of a starting-point, from which it spreads so as to 

 form two symmetrically placed bilateral organs; at the junc- 

 tion of the medulla and the cord the cells pass round to the 

 dorsal side, but here (fig. 4, PI. XXV) they are no longer com- 

 pact and thick, but isolated and scattered. We are here clearly 

 on the fringe of the compact organ, isolated cells from which 

 are found far down the spinal canal. 



At the termination of the conus post-commissuralis, in the 

 deep ventral fissure where alone anything approaching to a hilus 

 is to be found, we see, as in fig. 13 e, PL XXVI, how the lines of 

 pigment in between these large cells are massed together; and 

 we see how the termination of the tube which I have spoken of 

 as the liver-duct is lost in among this pigment mass at the very 

 place where it ought to come to the surface if the cells of this so- 

 called arachnoidal tissue are in reality the cells of the so-called 

 liver of the Crustacean-like ancestor, and this duct is the 



