454 Avery E. Lambert. 



paper on the morphology of the insect head, states that, in his opinion, 

 most of the recent investigators incline to recognize that the pre-oral 

 portion of the cephalic plate of insects is composed of three parts, or 

 segments. 



Assuming this to be the case, it appears that there are certain lines 

 of comparison by which the Arachnid brain can be homologized with 

 the brains of insects. 



1. The rudiment of the brain is laid down on the yolk as a broad 

 cephalic plate in both Arachnids and Insects. 



2. This plate, in both groups, presents three transverse segments, 

 anterior to the stomodseal depression, which form the foundation of 

 both the cerebral and optic ganglia. 



3. Each of the pre-oral segments, in Insects and Arachnids, pre- 

 sents a cerebral and an optic ganglion, and a marginal limb which 

 bears the retinal elements — the optic plate. 



4. The simple eyes of insects arise, as do the lateral eyes of 

 spiders and scorpions, in association with invaginations which appear 

 on the lateral and anterior margins of the cephalic plate. These 

 invaginations form important parts of the optic lobes in both groups. 



VIII. Geneeal Considerations. 



It would appear from the foregoing discussion that the brain of 

 the Arachnida presents a decided advance in complexity of struc- 

 ture, and in the method of its development, over that of the Insecta ; 

 or, indeed, any other group of the Arthropoda. 



The method by which the cerebral vesicle is formed, and the rela- 

 tion of the anterior median eyes to this structure, bears a striking 

 resemblance to the formation of the cerebral vesicle, and the growth 

 of the pineal eye, in vertebrates, as has been pointed out by Patten 

 in his paper on the Origin of the Vertebrates. 



It has been objected that the presence of two median eyes, which 

 are separate structures in the Arachnid head, would necessitate a 

 double origin for the pineal organ in vertebrates, no trace of which 

 can be said to exist. This objection has lost some of its force since 

 the publication, by Locy, of a paper in which he shows that, in the 

 Elasmobranchs, the pineal eye is formed by the union of two inde- 



