506 THE SENSES AND 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 
embryo 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.—KOlliker’s view, that the eyes of Invertebrates 
generally arise entirely froin the ectoderm, is very generally 
received, although both Claparéde and Weismann distinctly 
state that in the compound eye of Arthropods the nervous 
elements grow from the supra-cesophageal 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 Bobretzky [Parker, 250], who, however, 
incorrectly derives the whole eye from the cerebral outgrowth. 
The evidence on which the received view rests, that the com- 
pound eyes of Arthropods are developed entirely from the 
superficial epiblast, breaks down, as willbe 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 
special retinal end organs are modified epiblastic cells, and 
this may be their origin; but whenever the development of 
retinal end organs has been actually traced, except perhaps in 
the imperfect visual organs of some Hydrozoa, they have been 
traced to the central or nervous epiblast, and not to the surface 
