VERTEBRATES FROM A ORTJSTAOEAN-LIKE ANCESTOR. 403 



Sections through this region show indications of a separa- 

 tion of the raphe into two parts — a ventral one which terminates 

 in the conns post-coramissuralis^ and a dorsal one which 

 continues without interruption as the raphe of the aqueduct. 

 The appearance (fig. 11, PI. XXV) is as though the ventral 

 part was formed from a separate small tube lying in the middle 

 line ventrally, while the dorsal part was formed by the com- 

 pression of the ventral portion of the large cephalic stomach 

 as already meutioned. In some cases the separation of this 

 ventral tube from the rest of the raphe is much more con- 

 spicuous than in others, and in one instance it was remarkably 

 evident. 



In figs. 13 a — 13 e, PI. XXVI, I give a selection out of the 

 series of sections, to show the formation of this tube. The 

 Ammocoetes from which these sections were cut was by no means 

 fully grown ; its length was not measured, but judging from the 

 size of the sections I should estimate it somewhere about 40 

 mm. in length. The sections are all through the epichordal 

 portion of the brain, and, as is seen by the diminishing size of 

 the notochord, are numbered in the head wards direction. 



In the first section of the series, fig. 13 a, the raphe of 

 the epichordal portion of the brain is seen to extend close to 

 the ventral surface, and we see from the shape of the section 

 the commencement of a constriction on each side of the 

 external surface of the white matter ; this constriction marks 

 the beginning of the separation from the rest of the brain of 

 that portion of nervous matter which forms the ventral projec- 

 tion known as the conns post-commissuralis. 



In the succeeding sections we see how this indentation be- 

 comes deeper and deeper, until at last, as in figs. ISd, 13 e, this 

 projecting mass is entirely separated from the rest of the brain 

 by the strongly pigmented tissue which surrounds the brain, 

 and marks out most clearly the extent of the lateral indenta- 

 tions. Further, the sections show clearly that this portion, 

 which is thus cut off, contains the continuation of the ventral 

 portion of the raphe in the shape of a tube whose walls are 

 marked out with the usual fat-globules, and whose lumen has 



