THE EYES. 



65 



a. 



nient of the Araneid eye in Agalena and Lycosa. His results differ in some 

 essential points from those of his predecessors, and, if confirmed, are of 

 importance for the comprehension of the Araneid eye. 



The anterior median eyes (principal eyes) are first seen as ecto- 

 dermal thickenings of the frontal region. In front of this thickening 

 an invagination appears (Fig. 35 A, a), which eventually carries in 

 the whole of the 

 thickened area (Fig. 

 35 B). The deep 

 sac thus formed is 

 directed posteriorly, 

 and becomes applied 

 to the ectoderm, 

 which now shows 

 a second thickening 

 (Fig. 35 B, I). In 

 further explanation 

 of this sta^e we 



O 



refer the reader to 

 Fig. 10 B, p. 14, 

 which illustrates the 

 very similar course 

 of development of 

 the median eye in 

 the Scorpion. Here, 

 as there, the ecto- 

 dermal (or hypoder- 

 mal) layer lying over 

 the sac forms the 

 vitreous body, and 

 secretes the lens on 

 its outer side (Fig. 

 35 B and C, I). The 

 aperture of the in- 

 vagination (a) closes, 

 and the optic vesicle 

 thus becomes cut off 



from the ectoderm. The cells of the already thickened (outer) wall 

 of the invagination lengthen distally, their nuclei taking up a position 

 furthest from the surface, as can be seen in Fig. 35 C. The rods are 

 then secreted at the distal ends, and thus lie external to or in front 



F 



Fio. 35. — Development of the principal eyes of Agalena naevin, 

 sagittal section (after Locy). a, optic invagination ; gl, vitreous 

 body ; h, hypodermis ; I, lens ; ■}»', post-retinal layer ; r, retina. 



