158 THE EYES OF ARTHROPODS. 



It is clear that, as regards compactness and economy of space, the change 

 is an advantageous one. 



The optic lobes of vertebrates have been forced into their present position 

 by the general trend of several growth forces which appear at an early embryonic 

 period in the arthropod head. We have already referred to some of these condi- 

 tions. Those that are most persistent and which most affect the position of 

 the optic ganglia are : i. the overgrowth of the neural crests, which tend to carry the 

 ganglia from the margins of the medullary plate toward the median line; 2. the 

 central location of the eyes on the neural surface of the head in many free swimming 

 arthropods and in ostrocoderms; 3. the tendency of the entire brain to move forward 

 beneath the integument, while the rostrum and other superficial neural structures 

 move backward. (Compare Fig. 46.) 



When the optic ganglia are once established in a median neural position be- 

 hind the hemispheres, the increasing size of the latter, and of the optic lobes 

 themselves, exaggerates still more the backward movement of the neural portion 

 of the ganglia and of the primitive cerebellar commissure. 



Thus the roof and sides of the mesencephalon represent the ganglia of the 

 compound eyes of arthropods that have worked back into the territory behind 

 the hemispheres by that struggle for space between growing organs which ad- 

 justs and readjusts, till each part falls into the place of least resistance. 



Whether the optic lobes helped in the closure of the old mouth, or the disap- 

 pearing mouth and rostrum made a place for the lobes, cannot be determined. 

 Doubtless these events are part of a general movement, where it becomes im- 

 possible to distinguish cause from effect. 



Dr. L. Griggs, working at Dartmouth, has been able to locate the optic 

 lobes on the margins of the forebrain region, in the open neural plate stage of 

 Amblystoma, and has followed their course backward and upward till they 

 reach their permanent position. The movements of the lobes, as he describes 

 them, afford a striking confirmation of the interpretation given above. 



Conclusion. 



1. The lateral eyes are homologous throughout the insects, Crustacea, 

 arachnids, and vertebrates. 



2. In the arthropods they develop historically later than the larval ocelli, and 

 from a more posterior segment, namely the first appendage bearing segment 

 behind the primitive cephalic lobes. 



3. In the arthropods the lateral eye placodes lie for a time on the lateral 

 margins of the cephalic lobes, close to the deep infoldings that form a part of the 

 brain. In vertebrates, the eyes at first have a similar position, but a precocious 

 enlargement of the cephalic lobes and the neural crests leads to the enclosure of 

 the compound eye placodes in the brain chamber, so that they appear to form 

 a part of the brain. 



