ORIGIN AND STRUCTURE OF THE HEAD 29 



Among those who recognize the fore-runner of the Ver- 

 tebrate eye in the so-called eye-spot at the anterior end of 

 the brainvesicle of Amphioxus may be mentioned MiiLLER 

 (1874), AyeRS (1890), who discovers traces already of a 

 bilateral symmetry and a tendency to bipartition in it, and 

 Haeckel (1895). BOVERl (1904), on the contrary, derives the 

 Vertebrate eye from the segmentally arranged pigment-spots 

 which are found, except in a few anterior segments, in the 

 ventral wall of the medullary tube. They each consist, 

 according to HESSE (1898 p. 36), of two cells, a cup-shaped 

 pigment-cell applied to a visual cell with a nerve-fibre, 

 embedded in the convexity of the former. In the visual cells 

 BOVERI sees the homologue of the rods- and cone-cells in 

 the Craniate eye. The transformation is imagined by him in 

 this way: that primarily one of the segments of the medulla 

 containing the visual cells, approaches the body-surface by 

 devagination and that both the walls of the thus formed eye 

 vesicle, when it passed into the optic cup, have differentiated 

 in such a way that in the outer one the pigment-cells dis- 

 appeared ; in the proximal one, on the contrary, the visual cells 

 have been lost. In this way the retina and the pigment- 

 layer of the eye have originated. JOSEPH (1904, p. 24) 

 rightly emphasizes the many difficulties connected with this 

 conception. 



It cannot be denied that all the above cited theories, 

 however ingenuous or fascinating, are of a purely specula- 

 tive nature. In general they are not based on convincing 

 evidence derived from comparative anatomy or embryology. 

 They cannot serve as an argument for the view that the 

 Craniate eye is to be derived from that of Acrania, an 

 assumption they take as granted. In these two groups the 

 eyes have nothing in common but their encephalogenetic 

 origin. 



Eyes of Annelids and Molluscs. — My theory permits us to 

 give an explanation of the cephalogenesis of Craniates, which 

 at the same time has the advantage of providing the solution 

 of the second problem, the phylogeny of the Vertebrate eye, 

 and which moreover is supported by valuable embryological 

 arguments. On the episphere of the trochophora — ^nd even 

 already in the protrochula-larva of mesenchymatous worms — 

 we find a pair of pigment-spots which are already present 

 before the cerebral ganglia have been formed. In the adult 

 worm they are found again on the prostomium. No doubt 



