DEVELOPMENT OF THE EYES OF AGELENA 127 



and would give rise to an impulse which would be transmitted to the nerve 

 fibre arising from the proximal end of the cell. Since both of these bodies 

 appear to be developed as a modification of or a secretion into the cell 

 substance, it seems probable that either one or the other could be further 

 developed so as to form the main refractile element or be suppressed, thus 

 giving rise to differences in the position of the rhabdite relative to the 

 nucleus without there being any inversion of the cell. 



A true inversion of the layers of the retina, however, occurs in the 

 early stages of development in the median eyes of the scorpion (G. H. 

 Parker) and in the median eyes of the spider Agelena (Kishinouye). These 

 will be described in Chap. 24, Figs. 249 and 250. 



Development of the Eyes of Agelena 



Agelena is a cellar-spider, the development of which has been studied 

 by Kishinouye (1891-4). He describes the formation of a thickening in 

 the wall of the blastoccele, namely the ventral plate, on which the first 

 rudiments of the future organs make their appearance. The plate soon 

 shows a series of transverse grooves, indicating a metameric segmentation. 

 In front there is an undivided cephalic lobe, behind there is a caudal 

 lobe, and between these is a region which is divided into five segments of 

 which the first bears the pedipalpi, the remaining four giving origin to 

 the walking legs. The cephalic lobe gives rise to two semi-circular lobes 

 on each of which a crescentic groove is formed by an invagination of the 

 ectoderm ; this resembles the grooves already described in Peripatus 

 (Fig. 76), which are developed also in the scorpion and Astacus. The 

 ectodermal invaginations become closed off from the surface by union 

 of their edges, and they give rise to the greater part of the brain. The 

 inner segments of the two crescentic tubes now unite to form the stem of 

 a T, the transverse arms of which are formed by the outer portions of the 

 grooves. The central eyes (Fig. 89, A, B) arise as two pits at the posterior 

 ends of the grooves, which when united form the stem of the T and belong 

 therefore to the hindermost segment. The lateral eyes on each side 

 originate as a simple ring-like pit in the ectoderm (Figs. 89, 90), which 

 becomes divided by a continuance of the process of invagination into 

 several deeper secondary pits, each of which becomes closed off from the 

 exterior by constriction of its opening. The floors of these pits are 

 converted into retinulae, their component cells becoming the visual cells. 

 Between the upper or distal ends of the visual cells refractile rods or rhab- 

 domes are developed, while their proximal ends become continuous with 

 the constituent fibres of the optic nerve. The roofs of the pits are formed 

 by the overfolding of the ectoderm and form the vitreous or hypoderm 



