MUSEUM OF COMPARATIVE ZOOLOGY. 105 



tion of the fibres of the optic nerve in the anterior eye as in Fig. 23. The post-retinal 

 nuclei still distinguishable, although considerably flattened. In the posterior eye the 

 retinal cells are separated from the "lentigen" by a distinct pre-retinal membrane. 

 Bacilli already well developed iu the anterior half of the eye. The tapetum reaches 

 to the external cuticula as well as to the internal cuticula, and contains several tape- 

 tal nuclei. The optic nerve and its radiating fibi'es are shown in the " post-retinal " 

 layer. 



Figs. 25-32. Diagrammatic figures to show a possible course of development of 

 the several layers of the simple eyes from the hypodermis. Consult the text, pp. 

 87-98. 



Figs. 25, 26, 30-32. To illustrate the development of eyes of the " pre-nuclear " 

 type. The primary bacilli, at the distal ends of the hypodermal cells, are colored yel- 

 low ; the secondary bacilli, at the (originally) deep ends of the cells, red. Optic- 

 nerve fibres are yellow ; they should have been continued up to the nuclei. Arrows 

 in Figs. 30, 31, indicate the directions of the light from two different sources. (The 

 letters.^, and P. were accidentally omitted.) 



Figs. 25-29. To illustrate the development of eyes of the "post-nuclear" type. 

 The primary bacilli (yellow) retained on one side of the eye ; replaced on the other 

 side by the tapetum (here assumed to have been developed from the remaining half 

 of the original retina). Optic-nerve fibres, red; they should have been continued 

 up to the nuclei on the side away from the bacilli. 



CONTENTS. 



I. Source and Relations of Retina. p^c, 



1. Historical Summary 49 



2. Discussion of the Conditions in — 



a. Arachnoids 55 



i. Myriapods 60 



c. Insects 63 



II. Pre-retinal Membrane and "Sclera" 67 



III. Tapetum 72 



IV, Development of "Post-Nuclear" Eyes in Agelena. 



1. Posterior Median Eye 75 



2. Lateral Eyes 82 



V. Optic-Nerve Termination 84 



VI. Theoretical: Causes of Inversion in Eyes with — 



1. Pre-Nuclear Bacilli 87 



2. Post-Nuclear Bacilli 94 



VII. Bibliography 99 



VIII. Explanation of Figures 101 



