No. I.] EYES OF ARTHROPODS. 20I 



with the permanently paired arrangement of the pigment patches 

 and retinophorcz of the ocelli. 



The ommateum, in the lo mm, stage, is divided into two 

 nearly equal parts by a shallow, longitudinal depression of the 

 surface, below which are a number of round, irregularly-arranged 

 nuclei (Fig.6,iz). The absence of cell-boundaries and the irregu- 

 lar arrangement of the nuclei in this part of the ommateum are 

 in sharp contrast with the regularly arranged ommatidia in the 

 adjacent parts. At the beginning of the pupal stages this 

 median cord of nuclei has disappeared, and its place is occupied 

 by well-formed ommatidia. 



In the 8 mm, stage, of which no drawings are given, the 

 ommateum is supplied with two nerve-branches, one going to the 

 dorsal and the other to the ventral part of the ommateum. 



In the pupae the optic ganglion which during the preceding 

 stage was an oblong and nearly flat mass of ganglionic cells, 

 by the continued growth of its peripheral portions, becomes 

 an enormous, almost spherical, ganglion. Its medulla is now a 

 thick, nearly hemispherical shell, with its convex face directed 

 toward the ommateum (Fig. 7, omd). 



The change in the shape of the ganglion has been caused by 

 the growth of its inner edges {gc'^) toward the brain, while, 

 at the same time, the anterior and posterior boundaries of the 

 inner edge grow in opposite directions completely around the 

 medulla and finally unite with each other on its ventral side. 



In median sections through both optic ganglia, one therefore 

 sees beneath the outer and inner medullae, a wedge-shaped 

 mass of cells (Fig. 7, infge.), which, at first sight, appears to 

 form a distinct ganglion. Such, however, is not the case ; it is 

 merely a ventral continuation of the anterior face of the optic 

 ganglion. The " Keilformiges " ganglion of Berger (7), and 

 the "couronne ganglionnaire," and ** ganglion en coin" of 

 Viallanes (15), are merely continuous parts of the cortical layer 

 of the optic ganglion, apparently bounded by the fibres con- 

 necting the optic, with the retinal ganglion. 



The fan-shaped row of nerve-fibres, with its two rows of ganglion- 

 cells, has now developed into a convex layer easily recognized 

 as the retinal ganglion (Fig. 7, rtg} and rt.g?). The coarse, 

 straight fibres of the preceding stage have enlarged between 

 the cells of the inner layer, into refractive, spindle-shaped thick- 



