430 MALL. [Vol. VIII. 



is much the same according to the complexity of the eyes. 

 Nowhere is the histogenesis of the artliopod eye shown better 

 than in Watase's paper. He gives a clear demonstration that 

 the origin of the various cells of the ommatidium is from the 

 same source. This also corresponds with the vertebrate retina. 

 The corneagen cells differentiate much as, and correspond 

 with the rods and cones. The rhabdomere corresponds with 

 the ganglion cells ; both give rise to optic nerve fibers. The 

 former may differentiate into only one layer ; the latter into 

 three layers. The various groups of cells of the retina may 

 be firmly bound together by what are no doubt functional 

 cells — the ganglion cell in Limulus, and the bipolar cell of the 

 inner nuclear layer of the vertebrates.^ No matter whether or 

 not we succeed in making a homology of the various histological 

 elements of various eyes the fact remains that a cell with a 

 single prolongation or a differentiation of cells makes the 

 visual unit. An ommatidium on the one hand and a rod 

 and cone, bipolar cell and ganglion cell on the other ; all 

 are complex modifications of the same simple cell with its 

 polarity retained. In one cell (rod and cone) the receiving 

 or the afferent side is more greatly modified ; in the other cell 

 (ganglion) the transmitting or efferent side is more highly 

 specialized. 



That the growing point of the retina is at its periphery is 

 further demonstrated by the fact that the newt's retina may 

 be completely regenerated from this point. Griffini and 

 Marchino^ demonstrated, by cutting the optic nerve, that the 

 retina degenerates completely with the exception of its periph- 

 ery or ora serrata. We have seen that this is the embryonic 

 or growing portion of the retina, and Griffini and Marchino 

 find that the whole retina as well as the optic nerve is regen- 

 erated from this point. This fact does not seem so remarkable 

 when we take into consideration that the spinal cord may be 

 regenerated by the multiplication of the cells about its central 



1 These bipolar cells have been well described by Emery, Atti della Societa 

 Italiana di Scienza Naturali, XVIII, 1876 ; by Dogiel, Archiv, f. mik. Anat., 

 Bd. XXII, XXIV u. XXXVIII ; and by Ramon y Cajal, Anatom. Anz., 1SS9. 



2 Griffini and Marchin6, Arch. Ital. de Biol., XII. 



