MUSEUM OF COMPAKATIVE ZOOLOGY. 101 



EXPLANATION OF FIGUEES. 



LETTERS. 



The following letters are used to designate respectively : — 



= iluscle. 



r^ Muscle, cut cross-wise. 

 = Optic nerve. 

 = Posterior. 



= Post-retinal cell-layer of the eye. 

 = Pre-retinal cell-layer. 

 = Retina. 

 = Tapetum. 



Figures 1-24 were all drawn, with the aid of the Oberhauser camera, to the same 

 scale (X 515 diam.) from halsam-mounted sections cut from objects stained in alco- 

 holic borax carmine (Grenacher's) and imbedded in paraffine. Figures 1-16 and 

 18-24 relate to Agclena noivia; fig. 17 to Thcridium tcpidariorum, C. K. Figures 

 1-16, 18, 19, 23, 24 are from preparations by Mr. W. A. Locy; figs. 17, 20-22 from 

 preparations by Mr. G. H. Parker. 



PLATE L 



Figs. 1-7. Median faces of successive sagittal sections from the left half of the 

 head of a young Agclena ncevia, about four days after hatching. The position of the 

 portion of the brain nearest to the eyes is indicated at en c. 



Fig. 1. The plane of the nearer surface of the section passes through the middle of 

 the anterior median eye, cutting its optic nerve obliquely. The latter emerges from 

 the retina immediately heneath. the " lentigen." The distal ends of the elongated 

 nuclei in the "lentigen " are scarcely discernible, not being sharply marked off from 

 the surrounding substance, nor so deeply stained as at their proximal ends. Behind 

 the anterior eye, and beyond its optic nerve, are the muscles which separate the pos- 

 terior median eyes, and then pass obliquely forward and downward, in part beneath 

 the anterior median eye, in part between it and the anterior lateraf eye (compare 

 Figs. 2 and 3). Beyond these muscles, and partly obscured by them, is the layer of 

 cells composing the median wall of the posterior median eye. The muscle-cells are 

 traceable through the " hypodermis " to the cuticula at the surface of the head. 



Fig. 2. This section embraces a large portion of the lateral wall of the anterior 

 median eye, and the middle region of the posterior median eye. In the latter there 

 are four well-marked regions, — post-retinal, tapetal, retinal, and pre-retinal. 



Fig. 3. The lateral wall of the posterior median eye is embraced in this section, so 



