MUSEUM OF COMPARATIVE ZOOLOGY. 103 



PLATE n. 



Fig. 10. Median face of a sagittal section through the anterior median eye of the 

 left side, several days after hatching. The bacilli have begun to appear, and the 

 fibres of the optic nerve are seen to be distributed to the retinal cells near their 

 nuclei, — between them and the forming bacilli. The flattened nuclei of the post- 

 retinal tract still indicate the presence of a distinct layer of cells behind the retina. 

 The distance between the place where the optic nerve emerges and the " lentigen " is 

 greater than at first. (Compare Fig. 1.) 



Figs. 11, 12. Two successive sagittal sections of the anterior and post, median eyes 

 of the left side (median face) and of the same age as the preceding. The tapetum 

 of the posterior eyes is already formed. 



Fig. 11. The optic nerve of the post, eye communicates with the middle of the 

 post-retinal layer. A large portion of the median half of the tapetum removed with 

 this section. The optic-nerve fibres of the anterior eye (not drawn) were distributed, 

 as in Fig. 10. 



Fig. 12. The general direction of the original infolding is still evident in the pos- 

 terior eye. The post-retinal layer is contmuous with the hj'podermis in front of the 

 eye, and the pre-retinal behind. The bacilli at the anterior edge of the eye are partly 

 obscured by an overlying portion of the tapetum, in which are to be seen the elon- 

 gated nuclei. Beneath the outline of the anterior eye the optic nerve of the anterior 

 lateral eye of the same side is cut obliquely. 



PLATE IIL 



Figs. 13-15. Lateral faces of three successive sagittal sections, through the lateral 

 eyes of the right side. From the same spider as Figs. 11 and 12. 



Fig. 13. Beyond the plate-like bacilli (consult the text, pp. 82, 83) is the tapetum 

 with its longitudinal, uneven fissure, flanked on either side by the nuclei of the reti- 

 nal cells, which are scanty immediately in front of the tapetum. 



Fig. 14. Section through the bottom of tlie canoe-shaped tapetum of the post, eye, 

 showing the fissure and some of the nuclei of the flanking retinal cells, as well as 

 some of the narrower marginal nuclei of the post-retinal layer. In the anterior eye 

 are to be seen the post-retinal layer and some of the cells of the retina. 



Fig. 15. Only the nuclei of the post-retinal layer indicate tlie posterior eye. The 

 tapetum of the anterior eye is cut lengthwise near its middle. All four layers are 

 distinguishable, the bacilli being alreadj'^ developed. 



Fig. 16. Lateral face of a sagittal section through the posterior and anterior median 

 eyes (the latter in outline) from the left side of a specimen one day after hatching, 

 showing the four tracts of the posterior eye before the appearance of tapetum or 

 bacilli. 



Fig. 17. Theridiiim tepidariorum, C. K., adult. Portion of a sagittal section — 

 lateral face — passing though the tapetum of the posterior lateral eye of the right side. 

 The outer border of the tapetum is obscured b}' pigment-granules of the post-tapetal 

 layer, extending outward to the oval outline. The tapetal plates are large and quite 

 regularly arranged; the tapetal fissure is broad and irregular in outline. 



