RETINA OE THE BLOWFLY. 
415 
expansions consisting of epiblast-cells. It was Granin who, ten 
years later, first made sections and discovered their real structure. 
He found three distinct layers — Weismann’s epiblastic layer; 
his own provisional layer, which covers it externally as a fine cellu- 
lar expansion, which resembles the amnion of a mammalian 
embryo in being continuous with the periphery of the disc, in 
covering its whole outer surface, and in enclosing a cavity 
between it and the epiblast of the disc ; and the mesoblastic 
layer, which fills the hollow cup-like cavity on the inner surface 
of the epiblastic layer, and which consists of a network of fine 
branching cells. 
Weismann’s own figure (52, plate xiii., 1. c .) shows clearly that 
his supposed optic nerve is the mesoblast of the disc. My own 
observations show that the nervous pedicle of the optic disc 
becomes atrophied and disappears, whilst the nervous retina is 
developed as a papilla in front of the original optic pedicle. 
In my former paper I described and gave figures of the manner 
in which a new retina is developed during the skin-shedding of 
the Cockroach ; the original nervous pedicle of the disc corre- 
sponds to the nerve of the first few facets of the eye. As the 
number of facets is far greater after each ecdysis, so a new retina 
is developed from the nerve-centres as a distinct papilla; the first 
formed nerve and retina at the same time undergo atrophy. 
I regard the original pedicle of the disc in the Blowfly (figs. 5, 
6, & 7, st.) as a rudiment. It exhibits few, if any, nerve-fibres and 
consists chiefly of connective neuroglia continuous witb the in- 
vesting layer of the rudimentary hemisphere. The spongy meso- 
blastic tissue which Weismann mistook for an expansion of the 
nervous pedicle of the disc consists of the elements from which 
the tracheal vessels and pigmented fringes of the dioptron and 
neuron originate. This tissue extends into the dioptron, but only 
between the ingrowing optogenic cells, which become first colum- 
nar and then elongated rods, dividing during the process to form 
the cone and the investing cells of the great rods, and separating 
from each other to enclose the central cavity of the cone and 
the great rod. Claparede long ago correctly described the 
manner of the development of the cones and great rods. 
Viallanes, like Weismann, but with less excuse, mistook the 
mesoblast of the disc for the optic nerve and believed that its 
fibres perforate the axes of the great rods. It is easy in thick 
