452 Avery E. Lambert. 



nerves can be accoimted for in this way. If this should prove to be 

 the case, it will appear that the nerves supplying these eyes are sim- 

 ply modifications of the ectodermal cells which originally serve to 

 connect the retina with the vesicle of the anterior optic sac; and, 

 what is more significant, the anterior median eyes maintain their 

 connection with the optic tracts of the brain throughout the whole 

 process of their formation. 



The Posterior Median, and Lateral Eyes. — The accessory eyes 

 appear somewhat later than the principal eyes as simple ectodermal 

 thickenings in the optic area (Fig. 32 ; p. m. e. and I. e.). A depres- 

 sion appears in the region of each optic thickening from which these 

 eyes are formed, the walls of which grow inward in such a manner 

 that the thickened portion, or retina, is completely covered. The 

 cells which have grown over the retina in this manner secrete the lens, 

 and have been called, in consequence, the corneal layer. 



The manner in which the nerves supplying the accessory eyes are 

 formed is not clear. The possibility that the connection of these eyes 

 with the optic lobes is obtained secondarily has to be admitted, 

 although there appears to be little doubt in the minds of those who 

 have investigated the subject that the connection of the lateral eyes 

 with the lateral optic tracts bears some relation to the formation of 

 the lateral optic vesicle. 



VII. Comparison of the Akaneid Brain with that of Other 



Arthropods. 



Any effort to homologize the brain of Epeira with that of other 

 Arachnids, or with the brains of Arthropods in general, is attended 

 with a great deal of difficulty on account of the divergent statements 

 of the different investigators who have studied the subject. 



That there is a similarity between the brain of the scorpion and 

 that of the spider is evident from the descriptions both of St. Remy 

 and of Patten. This similarity is to be found not only in the form 

 of the adult brain, but also in its method of development. Patten 

 found three pre-cheliceral segments in the cephalic plate of the spider, 

 each segment consisting of a cerebral ganglion, an optic ganglion, 

 and an optic plate. He also found the same condition to hold true oi 

 the cephalic plate of the scorpion. 



