Procephalic Lobes of Epeira Ciiierea. 453 



The optic tracts in the spiders and in the scorpions arise in connec- 

 tion with an anterior optic, and two Literal optic, invaginations. In 

 both forms these invaginations give rise to homologous parts of the 

 optic lobes. In both the spiders and the scorpions the character of 

 the two sets of eyes is similar ; the median eyes being formed by a 

 process of infolding which results in the inversion of the retinal ele- 

 ments, in both forms the median eyes being formed, tirst as simple 

 ectodermal thickenings on the outer borders of the cephalic plate, 

 which, later, become folded over the head to form a cephalic fold that 

 bears the anterior median eyes on its posterior margin. 



Other investigators, however, do not all coincide with Patten in 

 his statement concerning the presence of three pre-cheliceral seg- 

 ments in the scorpion. Brauer and Lankester, whose papers appeared 

 before Patten's, report, the former, two such segments, and the latter, 

 one. McClendon, working under the direction of Wheeler, found 

 two segments in the species upon which Patten worked. Police 

 seems to have established the presence of two pre-cheliceral segments 

 in Euscorpius italicus. 



This divergence of opinion makes the question of comparison one 

 of great difficulty. In Epeira the three pre-cheliceral segments are 

 clearly distinguished, particularly in the earlier stages. From the 

 closeness of the relationship of the two forms, one would expect to 

 find the same number in the scorpion. Patten's figures of the 

 cephalic plate of Buthus would seem to make it evident that this 

 number exists in that form. 



An attempt to homologize the brain of the spider with that of 

 insects is attended with even greater difficulty. Patten records the 

 presence of three pre-oral segments in Acilius; the first of these 

 segments is less distinct in this form than in either the spider or the 

 scorpion. Wheeler, in his paper on Doryphora (41), believed that 

 he had found three pre-oral segments in that form ; but he was led 

 to change his conclusion in a later paper on the morphology of the 

 insect brain (42), in which he states that only two such segments 

 exist. 



Viallanes, on the other hand, finds evidence that three pre-oral 

 segments are present in the brain of Mantis; and Ilolingren, in his 



