YIII 



AETHEOPODA 



233 



formed by the outer portions of the grooves. The walls of the tubes 

 form the substance of the brain, and this organ shows a distinct 

 division into three segments. Of these the most anterior is formed 

 from the cross beam of the T, the other two form the main stem. 



The central eyes arise as pits just behind the posterior ends of the 

 brain grooves, they belong, therefore, to the hindermost segment, 

 while the lateral eyes are situated even still further behind this 

 point. Patten's attempt to show that 

 a pair of eyes belong to each segment 

 is, therefore, unjustified. 



The terms anterior and posterior 

 must be used with reference to the 

 mutual positions of the segments as 

 they lie in the anterior portion of the 

 ventral plate. By the growth in length 

 of the ventral ectoderm the segments 

 become pushed up round the anterior 

 end of the animal on to its dorsal surface, 

 so that what was anterior on the ventral 

 surface becomes posterior on the dorsal. 

 Thus the median eyes attain a position 

 behind the lateral eyes, although the 

 latter are morphologically posterior to 

 them. 



The lateral eyes on each side 

 originate as a simple ring-like pit of 

 ectoderm which becomes divided by the 

 continuance of the process of invagina- 

 tion into several deeper secondary pits, 

 and each of these becomes closed off 

 from the exterior by the constriction 

 of its opening. The floors of these 

 secondary pits are directly converted 

 into retinulae, their component cells 

 becoming visual cells. Between the 

 upper ends of the visual cells, rhabdomes 

 or visual rods are formed, whilst their 



lower ends are directly converted into nerve fibres which become 

 connected with the brain. The roofs of these pits are formed by over- 

 folding of the ectoderm, and constitute the viteUigenous cells which 

 secrete the crystalline bodies between them, whilst over all the 

 general cuticle is continued. The cuticle, which is of course secreted 

 by the ectoderm, is thickened where it covers the eye, forming there 



^ ^The central eyes have a different fate. When the vesicles out of 

 which they develop become closed, the last trace of the opening of 

 each vesicle is situated posteriorly, and each vesicle, therefore, consists 

 of an upper and an under wall with a slit-like cavity between them. 



FiQ. 183. — The condition of the 

 brain in the embryo of Agelena 

 after reversion has talcen place. 

 (After Kishiuouye.) 



A, frontal section of the brain and 

 adjacent structures. B, diagram of tlie 

 brain ; 1, 2, 8, the three segments of the 

 brain ; c.gr, cephalic groove (now closed 

 to form a tube); ch.g, cheliceral gan- 

 glion ; l.v, lateral vesicle ; o.c, rudiment 

 of central eye. 



