RELATION OF MEDIAN TO LATERAL EYES 



371 



to the sagittal plane a little in front of the centre of the shield ; they are 

 two in number and always symmetrically placed. The lateral eyes form 

 two isolated groups one on either side at the edge of the shield, where 

 its anterior border meets its lateral margin. In different genera the 

 number of eyes in each group varies from two to seven. Two kinds of 

 lateral eyes can be distinguished, the larger or " principal " and the 

 smaller or " accessory." No essential difference exists between these 

 two groups. As in the spiders, the vitreous and retinal layers are separate 

 (Fig. 251). These apparently two-layered eyes were described by 

 Lankester and Bourne as diplostichous. Locy and Patten, however, 

 recognized that these eyes are in reality three layered or triplostichous. 

 Patten (1886) also claimed that the median eyes of scorpions were formed 

 from a cup-like involution of the ectoderm. Parker fully confirmed 



Fig. 254. — A — Section through Early Stage of One of the Developing 

 Median Eyes of Agelena. B — Section through a Later Stage of a 

 Developing Median Eye. (After Kishinouye.) 



br. 3 : third lobe of brain. 

 ch. g. : cheliceral ganglion. 

 oc. m. : mouth of ocellar pit of median 

 eye. 



pr. : post-retinal layer. 

 r. : retina. 

 vit. : vitreous. 



these earlier observations, and showed that inversion of the retina takes 

 place during the development of the median eyes of the scorpion, Centrums, 

 as has been demonstrated in the median eyes of the spider Agelena by 

 Locy and Kishinouye (Fig. 254, A), but in the scorpion the two 

 optic pits or sacs are united by a common stalk (Fig. 253, B, C, D, E, 

 p. 370), whereas in the spider they appear as independent involutions. 

 In the scorpion the fibres of the optic nerve arise at first from the super- 

 ficial surface of the retinal layer (Fig. 253, D, E, F), and pass out beneath 

 the superficial layer of the fold of ectoderm which covers the retina 

 (Fig. 253, A, pr.r. and F, «./.). This superficial layer of the fold is known 

 as the pre-retinal layer, and gives rise to the vitreous or lentigen stratum. 

 During the later stages of development there is little change in the point 

 of exit of the fibres of the optic nerve ; it simply shifts from the embryonic 



