RELATION OF MEDIAN TO LATERAL EYES 369 



unit systems or ommatidia tend to elongate still further and undergo 

 further specialization. In some cases this elongation of the ommatidia 

 is attended by cell division, so that the retinal layer becomes changed 

 from the primary single layer of epithelial cells to one consisting of two 

 or more layers of cells, while in others the increase in depth is considered 

 to be brought about by simple elongation of the cells, which extend the 

 whole distance from basement membrane to cuticle (Patten). In the eyes 

 of many of the Crustacea there is an extension outward so that the retinal 

 layer becomes raised on the summit of a movable stalk which contains a 



Ch P 



SSEELSOC 





Fig. 252. — Longitudinal Section of Sphenodon Embryo (II, Stage S 1 ). 

 (After Dendy, showing Pineal Region.) 



C. Ab. : commissura abberans. 

 C.P. : commissura posterior. 

 C.S. : commissura superior. 

 Ch. P. : choroid plexus. 

 D.S. : dorsal sac. 

 Par. : paraphysis. 



Par. PI. : parietal plug. 



P.E. : pineal eye. 



P.R. : pineal recess. 



P.S. : pineal sac. 



S.C.O. : subcommissural organ. 



S.O.C. : supraoccipital cartilage. 



series of secondary neurones ; these form one, two, or three optic ganglia 

 which connect the retinal ganglion with the supra-oesophageal ganglion 

 or fore-brain. Intermediate conditions are found between the more 

 primitive and less specialized types in which only one optic ganglion is 

 present, as in Branchipus, and the more highly evolved genera such as 

 Astacus, in which three are present. 



In the median eyes (parietal or entomostracan) the development of 

 the placodes does not extend beyond the one-layered simple type of 

 retina, and in some the visual layer becomes inverted and the placodes of 

 opposite sides become fused in the median plane. 



1 See reference, p. 262. 

 24 



