RETINA OF THE BLOWFLY. 



415 



expansions consisting of epiblast-cells. It was G-anin 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., I. c.) shows clearly that 

 his supposed optic nerve is the raesoblast 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 with 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 tlie 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 



