VERTEBRATES PROM A CRUSTACEAN -LIKE ANCESTOR. 389 



of the cephalic stomachj but also of the mouth and oesophagus, 

 is much more clearly visible than in the higher forms. 

 Another point worthy of note is the shape of the cavity of 

 this median tube ; it is always either round or compressed 

 into a horizontal slit, never into a vertical slit ; showing that 

 the pressure to which it has been subjected by the growth of 

 the nervous system is not lateral, but is in the dorso-ventral 

 direction, as would naturally follow if the oesophagus of such 

 a form as Limulus were compressed by the growth of the infra- 

 oesophageal ganglia under which it lies. 



Embryologically, this tube and its termination form the 

 anterior neuropore, which, as Ahlborn (4, p. 333) has pointed 

 out, must be looked for in the neighbourhood of the hypo- 

 physis rather than of the epiphysis. 



Sect. 3. — The Relation of the Infra-oesophagealand 

 Thoracic Ganglia to the Walls of the Cephalic 

 Stomach. 



If the choroid plexuses are the free dorsal walls of the cephalic 

 stomach thrown into folds, it follows that the continuation of 

 the tissue of the choroid plexuses which lines the cavities of 

 the brain must be the ventral portion of the stomach walls. 

 Also the nervous masses which lie on the outer side of the 

 lining epithelium, and form the grey and white matter of the 

 brain, must, on the same theory, be composed of elements 

 arranged in the same way, and of the same intimate structure 

 as the nerve-masses which form the supra oesophageal, infra- 

 oesophageal, and thoracic ganglia of the Crustacean-like ancestor. 

 Such appears to me to be undoubtedly the case ; and although 

 I must wait for the proof of the complete homology of the 

 different parts in the two nervous systems until the completion 

 of this series of papers, yet it is well worth while to point out 

 how strikingly the microscopic appearance of the brain of the 

 Ammocoetes bears out the theory. I will first deal with the 

 epichordal portion of the brain, i.e. with the relations of the 

 infra oesophageal and thoracic ganglia to the stomach walls ; 



