ON THE OFJGIN OF VERTEBRATES. 



249 



The resemblance throughout is so striking as to force one to 

 the conclusion that the retina of the vertebrate eye is a compound 

 retina composed of a retina and retinal ganglion of the type found 

 in Arthropods. From this it follows that the development of the 

 vertebrate retina ought to show the formation of (1) an optic 

 plate formed from the peripheral epidermis and not from the 

 1 )rain ; (2) a part of the brain closely attached to this optic plate 

 forming the retinal ganglion, which remains at the surface 

 when the rest of the optic ganglion withdraws ; (.3) an optic 

 nerve formed in consequence of this withdrawal, as the connec- 

 tion between the retinal and cerebral parts of the optic ganglion. 



Slip. Segment 

 I 



Ant 1 



Ant II 



Inf. Segment 



Fig. 7. — The brain of Sphseroma serratum (after Bellonci). Ant. I. and Ant. II. 

 nerves to 1st and 2nd antennre. f.hr.r., terminal fibre layer of retina; 

 Op. g. I., first optic ganglion ; Op. ij. II., second optic ganglion ; O.v., optic 

 nerve fibres forming an optic chiasnia. 



This appears to me exactly what the developmental process 

 does show according to Gotte's ^ investigations. He asserts that 

 tlie retina arises from an optic plate, being the optical portion 

 of his Sinnes-platte. At an early stage this is separated by a 

 furrow (Furche) from the general mass of epidermal cells which 

 ultimately form the brain. This separation then vanishes, and the 

 retina and brain mass become inextricably united into a mass of 

 cells, which are still situated at the surface. By the closure of 

 the cephalic plate and the withdrawal of the brain away from 

 the surface, a retinal mass of cells is left at the surface connected 

 ^ Enlwick. Gesch. d. Unke, Leipzig, 1875. 



