ESTERLY: EUCALANUS. 33 
fortsetzt.”’ Grenacher (79) makes a similar statement. Both Claus 
(91) and Hesse (:01, :02), however, state that the median eye of 
Crustacea lies outside the ectoderm. 
My preparations show that this statement quoted from Claus (91) 
is only partly correct. ‘The optic nerve is surrounded by a very dis- 
tinct membrane, which becomes blue in Mallory’s stain, and is there- 
fore to be regarded as composed of connective tissue, and is probably 
of mesodermal origin. The membrane enveloping the optic nerve is 
continuous with the one around the brain (Plate 1, Fig. 4), but has 
nothing at all to do with the neurilemma of the nerve fibres (Plate 5, 
Fig. 47). But the discussion of the relation between this sheath of 
the optic nerve and the eye may be deferred for the present. 
In its relation to the hypodermis, or ectoderm, the ventral part of 
the median eye corresponds in every way to the first of the retinal 
types mentioned by Parker (91), though Hesse (:02) states that the 
whole median eye is detached from the ectoderm. Both cross and 
sagittal sections of the eye (Plate 1, Figs. 3, 6; Plate 5, Figs. 44, 
48) show with perfect clearness that the ventral portion of the trip- 
artite eye is merely a thickened region in the ectoderm (h’drm.), and 
has maintained its superficial position permanently. ‘The hypodermis 
and retinal cells are adjacent; in other words, the retina and ectoderm 
are continuous. ‘The basement membrane of the ectoderm may be 
traced in sagittal (Plate 5, Figs. 44, 48) and in cross sections (Figs. 
49, 50) continuously from a region entirely outside the retina, over 
(dorsal to) the basal plate of the eye, and on to the ectoderm as such 
again. ‘This basement membrane is very delicate, but visible with 
perfect clearness in any preparation, though Mallory’s stain shows it 
most distinctly. In this stain the membrane does not become blue, 
and so should be regarded as of a different nature from that envelop- 
ing the optic nerve and brain. 
Such nerves as leave the ventral eye through the basal plate, must 
penetrate the basement membrane of the hypodermis as well as the 
basal plate, since the plate is a product of the cells composing the 
retina, and these are plainly specialized ectodermal cells. ‘The nerve 
fibres which in passing from the visual cells do not penetrate the basal 
plate pass through the basal membrane only. 
The relations of the dorsal components of the eye and their envelop- 
ing membranes are not so easily made out as in the case of the ventral 
eye. Each of the paired eyes is surrounded by a delicate sheath 
(Plate 5, Figs. 46, 50), but in the adult condition it is evidently not 
related to the ectoderm as in the ventral eye. Such illustrations as 
