506 THE SENSES .1X1) SENSORY ORGANS. 



pineal eye are derived from the primitive brain vesicle, and 

 appearances are certainly in favour of the view that the lens 

 when present is developed from the same vesicle ; yet I think 

 it by no means certain that such is the case. The difficulty of 

 finding a primitive involution of the surface ectoderm in the 

 t-nibryo would necessarily be great, even if at one time or other 

 it exists, and I regard the mere apparent continuity of the 

 retinal and lens cells as of little value as evidence of a similar 

 origin for both. The existence of a modified scale and of a 

 parietal foramen points to a surface modification, and I doubt if 

 the last word has been said on this point. It is, moreover, one 

 of no import in relation to my contention, that in Arthropods 

 the retina is developed from the brain, for in these it is certain 

 that the refractive media are dermal structures. 



Epiblastic Eyes. Kollikcr's view, that the eyes of Invertebrates 

 generally arise entirely from the ectoderm, is very generally 

 received, although both Claparede and Weismann distinctly 

 state that in the compound eye of Arthropods the nervous 

 elements grow from the supra-oasophageal ganglia, a statement 

 which is fully confirmed by my researches. Moreover, Kolliker 

 himself is evidently in some doubt on the subject, as he has 

 classed the compound eyes of Astacus as eyes developed partly 

 from the medullary plate, and partly from the central nervous 

 system, like the paired eyes of Vertebrates. In this he has 

 apparently followed I-3obretzky [Parker, 250] , who, however, 

 incorrectly derives the whole eye from the cerebral outgrowth. 

 '1 he evidence on which the received view rests, that the com- 

 poiiiid eyes of Arthropods are developed entirely from the 

 superficial epiblast, breaks down, as will be seen in the sequel, 

 as soon as the true developmental history of the parts is more 

 completely known. 



Origin of the Retinal End Organs. It is usually held that the 

 sp'-rial retinal end organs are modified epiblastic cells, and 

 this ma}- be their origin; but whenever the development of 

 retinal i-iid organs lias been actually traced, except perhaps in 

 tin- imperfect visual organs of some Hydrozoa, they have been 

 traced to the central or nervous epiblast, and not to the surface 



