VBRTEBEATBS FEOM A ORUSTAOEAN-LIKE ANCESTOR. 419 



matter iii the infra-a3sophageal region has in the course of its 

 growth become closely united with the walls of the cephalic 

 stomach, and has compressed and folded those walls in various 

 ways in different regions, it is reasonable to suppose that 

 here also the elements of the supra-oesophageal ganglia will 

 be found in close connection with the walls of the anterior 

 termination of the cephalic stomach, and will also surround 

 diverticula produced by the compressing growth. 



The mere inspection of the dorsal view of the brain of the 

 Ammocoetes (fig. 1) almost compels us to recognise the supra- 

 oesophageal ganglia in the separate masses of nervous matter 

 which are situated in front of the folds of the choroid plexuses, 

 and form the cerebral and olfactory lobes connected by means 

 of the optic thalami with the ganglia habenulse, which in their 

 turn are connected with the pineal eyes. 



Here, also, we find, as in the lower regions of the brain, that 

 the walls of the cavity are partly free from the invasion of 

 nervous matter, and partly form the lining epithelium of the 

 nervous masses which are lying outside them. Thus, as already 

 mentioned, we can trace the free anterior wall of the cephalic 

 stomach in the middle line in front of the choroid plexus ii 

 as forming the recessus infrapinealis (Ahlborn), then the 

 choroid plexus i which bridges over the space between the two 

 optic thalami (Hirnschlitz), continuing onwards as the lamina 

 terminalis to reach the ventral side of the brain, where it 

 forms a bulging known by the name of the recessus chias- 

 maticus. 



On each side of the lamina terminalis lie the simple 

 olfactory lobes and cerebral hemispheres of the Ammocoetes. 

 They have grown round the anterior wall of the cephalic 

 stomach, so as to include two diverticula called by Ahlborn the 

 lateral ventricles of the brain. In fig. 6, PL XXV, I give a section 

 through an osmic preparation of the brain of an Ammocoetes 

 100 mm. long. The section is a horizontal one, and is one of 

 a consecutive series through the whole brain. It shows the 

 olfactory nerves passing from the olfactory lobes, and the 

 lateral ventricles of Ahlborn connected by the ventriculus com- 



