ox THE DEVELOPMENT 01" ARANEIXA. 77 



(PJ. XVI, fig. 49. L). The nerve cLjes not enter the p<jstenor median 

 eyes even a few days attei' the hatching of the embryo. Probably 

 the nerve is sent out from tlie retina from the anterior (morphoJooi- 

 calJy inner) surface of it, as this is the case in the aduU. The deve- 

 lopment of the posterior median e^'es is comparatively slow. They 

 are homologous with the median eyes of scor|)ions, as the develop- 

 ment is quite the same. 



The three remaining pairs of the eyes or ' Xebenaugen ' of 

 Bertkau* are formed later than the posterior metlian eyes ; but their 

 development is completed earlier. They arise from ring- like depres- 

 sions of the ectoderm (fig. 50). The walls surrounding these 

 depressions grow over them and finally meet (fig. 51). The spot 

 where the walls meet is one-cell layered. This spot gi-adually ex- 

 tends to a certain extent and forms the vitreous body which is 

 characterized by elongated cells (figs. 51, 54, vit). The growth of the 

 walls of the depressions is not uniform in every direction and there- 

 fore the point of closure may not correspond with the centre of de- 

 pression. Thus the ' Nebenaugen ' are also formed from ectodermic 

 sacs ; but these sacs are diiferent from the sacs of the posterior median 

 eyes. While it is the anterior wall of the sac that becomes the retina 

 in the posterior median eyes, it is in the case of the " Nebenaiigen' 

 the posterior wall of the sac, which, forming a central elevated })ortion 

 thicker than the anterior wall and siu'rounded by a ring-like depres- 

 sion, gives rise to the retina (figs. 50-54, R). Also retinal cells 

 elongate posteriorly instead of anteriurlij, as in the posterior median 

 eyes, and form nerve fibres (figs. 50, 51, A^). These nerve fibres are 

 subsequently connected with the fibrous portion of the brain. The 

 retinal portion is cut ofi:' from the general ectoderm at about tlie time 



* Burtkau, lue. cit. 



