Studien zur Urgeschichte des Wirbelthierküi'pers. X. 441 



nicht verwirft, so anerkennt er doch, dass die ersteren drei beträcbt- 

 liche Reductionen erlitten haben , und sieht sowohl im Auge der Tuni- 

 caten, wie auch in dem der Cyclostomen so starke Anzeichen der De- 

 generation, dass er auch der oben erwähnten LANKESTER'schen Hypo- 

 these die Wahrscheinlichkeit abspricht. Die Hauptschwierigkeit scheint 

 ihm in dem Umstände zu liegen, dass die Retina der Vertebraten ein 

 Stück des Vorderhirnes sei, und er giebt dafür folgende Erklärung: 



y)Tliefact of the retina heing derwed front the fore-braiu may per- 

 haps he explained in the same way as has already heen attempted in the 

 case of the retina of the Crustacea; i. e. hy supposing that the eye was 

 evoloed simultaneoudy with the fore-part ofthe brain. Tlie peculiar pro- 

 cesses which occur in the formation of the optic vesicle are more dimenìi 

 to elucidate ; and I can only suggest that the devehpment of a primary 

 optic vesicle, and its conversioìi itito an optic cup, is due to the retinal par t 

 of the eye having heen involved in the infolding ivhich gave rise to the 

 canal of the centrai nervous system. The position of the rods and cones 

 on the posterior side of the retina is satisfactorily explained hy this hypo- 

 thesis. hecause the posterior face of the retina is the originai external 

 surface of the epidermis^ which is infolded in the formation of the hrain, 

 so that the rods and cones are, as may he anticipated, situated on lohat 

 is morphologically the external surface of the epihlast of the retina. 



» The difficulty of this view arises in attempting to make out how 

 the eye can have continued to he employed during the graduai change of 

 position xohich the retina must have undergotie in heing infolded ivifh the 

 hraiti in the manner suggested. If liowever the successive steps in this in- 

 folding were sufficiently small, it seems to me not impossihle that the eye 

 might have continued to he used throughout the whole period of change, 

 and a transpareticy of the tissues, such as hankester suggests, may have 

 assisted in rendering this possihle. 



y^The difficulty ofthe eye to continuing he in use lühen under g oing stri- 

 king changes in form is also involved in hankester'' s vieto in that if\ as I 

 suppose, he starts from the eye of the Ascidian tadpole with its lenses 

 turned towards the cavity of the hrain; it is necessary for him to admit 

 that afresh lens and other optical parts of the eye hecame developed oti 

 the op pò site side ofthe eye to the originai lens; and it is difficult to 

 understand such a change, unless we can helieve that the refractive media 

 on the two sides were in Operation simultaneously . It may he noted that 

 the same difficulty is involved in supposing, as I have clone, that the eye 

 of the Ascidian tadpole was developed from that ofthe Vertebrate. I 

 should however he inclined to suggest, that the eye Ì7i this case ceasedfor 



Mittheilungen a. d. Zoolog. Station zu Neapel. Bd. VI. 3(j 



