DEVELOPMENT OF THE EYE. 



151 



(vitreous body), a thick layer of retinal cells, and a layer of pigment 

 behind the whole. In the retina, the cell nuclei lie in front of the 

 rods ; these latter therefore belong to the posterior part of the cells, 

 and thus come into direct contact with the pignient-layer (Fig. 70, st). 

 The nerve-fibres become connected, however, with the outer ends of the 

 visual cells, so that here also the same conditions prevail as are found 

 in the posterior middle eyes and the lateral eyes of the Araneae 

 (Fig. 34, p. 64). This last point, which seems to be implied in the 

 description given by Hoek, has recently been established by Morgan 

 (No. 12). 



The ontogenetic stages, as well as the adult structure, closely 

 resemble those of the Arachnida, as may be seen by comparing 

 Fig. 70 with the ontogenetic stages of the eyes of the Scorpiones 

 and the Araneae illustrated in Fig. 10, p* 14, and Fig. 35, p. 65. 

 In Fig. 70, an invagination extending from one side below the hypo- 

 dermis is suggested. The retina and the pigment-layer thus arise, 

 and out of the superjacent hypodermis the layer forming the vitreous 

 body, which secretes the lens on its outer side. An inversion thus 

 takes place in the formation of the eye, and its innervation would 

 be from the first explicable in the same way as that of the eye of 

 the Araneae. 



In earlier stages in the development of the eyes, an invagination 

 is not so distinctly recognisable 

 as in the eyes of the Araneae. 

 The different layers of cells lie 

 somewhat close to one another, 

 and Morgan assumes that no 

 actual (complete) invagination 

 takes place, but rather that 

 new cells are continually being 

 added to the eye from the 

 point of ingrowth, and that 

 thus finally the layers, like 

 those in the Arachnid eye, are 

 formed (Fig. 70). A thicken- 

 ing of the hypodermis, which 



appears laterally to the eye, perhaps yields the new cell-material. 

 This hypodermal thickening recalls the one found near the Crus- 

 tacean eyes and those of Limulus (Vol. II., pp. 280 and 359). 



The development and the structure of the Pantopodan eyes suggests through- 

 out a comparison with those of the Arachnida. Morgan's statement that the 



ettr r 



Fig. 70. — Longitudinal section through one of 

 the posterior eyes of the larva of Tanystylum 

 (after Morgan), c, cuticle ; cct, hy, ectoderm 

 (hypodermis) ; gl; vitreous body ; pi, pigment- 

 layer ; r, retina ; st, rods. 



