234 



INVERTEBKATA 



CHAP. 



From the upper, not the lower of these two layers, the visual cells 

 are developed, aod from the ectoderm which covers the whole sac, the 

 vitelligenous cells are formed. It follows that what were originally 

 the outer ends of the visual cells are turned away from the light, 



Fig. 184. — Sections through the developing eyes of Agelena. 



(After Kishinouye.) 



A, section through early stage of development of the median eye. B, section through later stage of 

 development of median eye. 0, section through invagination from which the lateral eyes develop. 

 ir3, third lobe of the brain ; e7i.(/, cheliceral ganglion ; w./, nerve fibres ; oc^ rudiment of median eye ; 

 oc.l, rudiment of one of the lateral eyes ; ^.'t\ post-retinal layer of cells ; r, layer of visual cells ; ot(, 

 vitelligenous layer. 



whilst their basal ends, from which the nerve fibres spring, are turned 

 inwards. The lower end of the sac forms the post-retinal layer (^.r, 

 Fig. 184). 



The nervous system before and during reversion is in the form 

 of a thickening of the ectoderm ; it now becomes detached from the 



Fig. 185. — Two sagittal sections through embryos of the spider Theridion inaculatum, in 

 two succeeding stages of development. (After Morin, from Korschelt and Heider. ) 



A, ventral nerve cord, showing ganglia. B, ventral ganglia fused to form a suboesophageal mass. 

 Jr, brain ; dil, dilator muscle of " stomach " (i.e. stomodaeum) : end, endodermic epithelium beginning 

 to cover the mass of yolk ; H, heart ; hep, liver ; proct, proctodaeum ; sh.o, suboesophageal ganglion ; ster, 

 stercoral pocket ; atom, stomodaeum or stomach ; v.g, ganglia of the ventral nerve cord. 



ectoderm and the ganglia of the ventral chain soon fuse into one 

 suboesophageal mass (Fig. 185, B). The characteristic poison gland 

 is developed as an ectodermal ingrowth in the base of the chelicera. 

 The stomodaeum develops rapidly, sloping upwards and backwards. 



