104 BULLETIN OF THE 



Figs. 18, 19. Two successive sagittal sections of the anterior lateral eye of the left 

 side. . Agelena ncevia, eight days after hatching. The tapetum does not reach the 

 external cuticula. 



Fig. 18. The distant surface of the section nearly coincides with the plane of the 

 tapetal fissure, only a small portion of its left half being shown, near the muscle-fibres, 

 mu'. The greater portion of the tapetum is seen from behind, and nearly perpendicu- 

 lar to its surface. It obscures the bacilli in front of it. There are five greatly elon- 

 gated tapetal nuclei stUl visible. 



Fig. 19. The nuclei of the "post-retinal" layer are large and closely packed. 

 Three tapetal nuclei are visible at the edges of the tapetum, which is viewed more 

 nearly edgewise than in Fig. 18. The bacilli are perpendicular to the corresponding 

 parts of the tapetum, and therefore do not all point to a common imaginary centre. 



PLATE IV. 



Figs. 20-22. Median faces of three successive sagittal sections through the right an- 

 terior and posterior median eyes, after the formation of pigment-granules has begun. 



Fig. 20. The position of the optic nerve of the anterior eye is sketched in from 

 the two preceding sections. It will be seen that its place of emergence is still farther 

 from the " lentigen " than in Fig. 10. The near edge of the tapetum in the posterior 

 eye shows two elongated tapetal nuclei. The pre-retinal (lentigenous) nuclei are dis- 

 tinguishable from the retinal nuclei by their deeper color, greater flatness, and more 

 granular appearance. 



Fig. 21. In the posterior eye five tapetal nuclei are visible, and in the "post- 

 retinal " layer the radiating branches of the optic nerve. The near edge of the bacil- 

 lar layer is cut; the nuclei of retina and " lentigen " appear as in the previous section. 



Fig. 22. In the anterior eye the post-retinal layer has apparently joined the retinal 

 layer. (Compare Fig. 20.) Many of the retinal nuclei present a peculiar vacuolated 

 appearance at their outward ends. The bacilli have attained a greater length than 

 in Fig. 10, their deep ends being covered by pigment-granules. The anterior eye is 

 joined by a muscular (?) band — inserted near the emergence of the optic nerve — to 

 the anterior margin of the posterior eye. In the latter the tapetum reaches the in- 

 ternal cuticula, but not the external cuticula; it contains four tapetal nuclei. Many 

 of the retinal nuclei are elongated, and taper at one end (compare the retinal nuclei 

 of the anterior- eye in Figs. 23, 24), probably indicating the region of their connec- 

 tion with a fibre of the optic nerve. The bacilli are evidently more numerous than 

 the retinal nuclei. 



PLATE V. 



Fig. 23. Median face of a sagittal section ; anterior and posterior median eyes. 

 The course of the optic nerve to the posterior eye sketched in from the preceding 

 sections. In the anterior eye the fibres of the optic nerve can be traced to the elon- 

 gated anterior ends of the retinal nuclei, which exhibit shapes dependent on this 

 union. 



Fig. 24. Lateral face of a sagittal section through the anterior and posterior 

 median eyes of the right side of the same specimen as the last figure. The distribu- 



