34 



THE ANCESTRY OF VERTEBRATES 



fused with the anterior ganglia of the ventral nerve chain, 

 from which the Vertebrate brain is derived. The eye rudi- 

 ments follow them and are incorporated with the brain 

 involution from which they grow out again to the surface 

 with an inverted retina. No such fusion of the cerebral 

 ganglia with the ventral nerve-chain needs to be assumed, 

 however, if we apply the principles of my theory to this case. 



prostornium., soma 



5tom 



p. card. 



.rchen 



Fig. U. Diagrams of an Annelid, an Acraniate Chordate before the formation of 

 tlie tail has begun and a Craniate Chordate with tail (cf. Chapter III). 

 a anus, a. p. animal pole, archenc. archencephalon, can. med. medul- 

 lary tube, near. p. neuropore, p. card, cardiac pore, p. near, neurenteric 

 pore, stom, stomodaeum. 



Homology of Craniate and Protostomian eyes. — Thus we 

 are led along a quite different path to a conclusion already 

 indicated by a consideration of the relation of the fore-end 

 of the notochord to that of the medullary tube, scil. that 

 part of the surface of the episphere or prostomium con- 

 taining the pigment-spots, has, so to speak, been annexed by 

 the forward extension of the medullary tube, thus forming 

 the praechordal part of the brain with the optic vesicles 

 which in Vertebrates have evidently developed along lines 



