128 



THE PINEAL ORGAN 



cells, while over all is formed a lens-like thickening of the cuticle, which is 

 continuous with the surrounding cuticle. 



The median eyes (Figs. 89, 90, A and B) are formed also as ectodermal 

 pits which become closed off from the surface by constriction of their 

 openings. The opening of each vesicle is situated behind, and when 

 closed each vesicle has an upper and a lower wall between which is a slit- 

 like cavity. From the upper of the two layers the visual cells arise, while 

 the lower wall gives origin to what Kishinouye describes as the " post- 

 retinal " layer (Fig. 250 B, Chap. 24, p. 362). The vitreous, as in the 



A B 



Fig. 89. — Brain of Embryo of Agelena showing the Position of the Median 

 Eyes and Lateral Vesicles. (After Kishinouye.) 



A — Seen from in front. 

 B — Seen from left side. 



c. gr. : cephalic groove, now closed. 

 ch.g.: cheliceral ganglion. 

 l.v. : lateral eye vesicle. 

 m.e. : median eye. 



Fig. 90. — Frontal Sections through Brain of Embryo Agelena. (After 



Kishinouye.) 

 c.gr. : cephalic groove. 



I.e. : thickening of ectoderm, which will form the lens of the lateral eyes. 

 l.v. : lateral vesicle. 

 m.e. : median eye. 



v.b. : vitreous body (lens of median eye). 

 1, 2, 3 : the three segments of the brain. 



