215 
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 MüLLER 
(1874), AYERS (1890), who discovers traces already of a 
bilateral symmetry and a tendency to bipartition in it, and 
HAECKEL (1895). BOVERI (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 (1838 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 — end 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 
