82 Fiederick H. Ivrecker, 



their distal end are extremely slender. The dark areas are the basal 

 thickenings of the visual rods. These soon, i. e. near the end of the 

 third stage, begin to project distad of their respective cells, together 

 forming a narrow, faintly staining band directly beneath the chitin 

 (Fig. 4). In succeeding stages one can easily follow the gradual ex- 

 tension of the rods. Perhaps a comparison of the figures showing the 

 various stages will give a better understanding of their future de- 

 velopment than would a verbal description. In the fourth stage the 

 rods increase their length very rapidly. The axial rods, which are the 

 first to appear, continue to be longer than the others but this does 

 not cause them to project beyond these. On the contrary the distal 

 ends of the axial rods are lower than those of the others, since hand 

 in hand with the elongation of the rods goes a sinking in of the 

 axial portion of the eye (Fig. 5). When the pigment begins to be 

 deposited during the latter half of the fourth nymphal stage, or 

 even earlier, the rods have attained practically their füll length, 

 the only further change being a gain in diameter. From the very 

 first the rods are, as in the completed eye, of a finely granulär 

 composition which does not stain readily in any of the stains used 

 except Bleu de Lyon. 



Almost simultaneously with the beginning of the visual rods the 

 neural fibers appear, noticible first on the axial cells. The proximal 

 ends of these cells now bend ectad so that the fibers converge toward 

 the .'extreme ecto-proximal portion of the base, from which point a thin 

 cord of fibers extends along the side of the head between the supra- 

 oesophageal ganglion and the body wall, as far as the base of the la- 

 teral eye, where it enters the brain (Fig. 4). About the nerve is a 

 sheath which, however, is not derived from the neurilemma of the 

 cerebral ganglion, although later with the increasing size of the nerve, 

 the cells of this neurilemma do envelop it. The primary sheath, if it 

 may be so termed, is derived from the neighbouring connective tissue. 

 The cells have large nuclei and at either end of these the cell bodies 

 are drawn out into long thin processes which are closely applied to the 

 sides of the nerve (Fig. 4). As more fibers appear a cushion of nervous 

 substance forms at the basal end of the retinal cells, but the size of 

 the nerve increases slowly until the latter half of the fourth nymph 

 stage. From then on it grows rapidly and attains such a size that, 

 apparently, the optic lobes are prolonged until tliey reach the base 

 of the eyes. 



During the latter part of the third and throughout most of the 



