No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 107 



belong to one segment — the protocerebrum. That such is 

 the case I am convinced from a study of the eyes in embryos 

 of Dytiscus vcrticalis, a form closely related to Acilius. 



As will be seen in the profile view Fig. 8, the optic ganglion 

 and optic plate of XipJiidiiim are at first folded back so as to 

 lie along side the deuto- and tritocerebrum. The antennal 

 furrow runs forward, separating the optic ganglion from the 

 brain but stops when it reaches the protocerebrum. The 

 value of this furrow as completely separating the second and 

 third brain-segments from the optic ganglion was overlooked 

 by Patten and myself: hence our false interpretation of the 

 structures lying laterad to it. 



I believe that I am justified in putting this new interpreta- 

 tion on the Coleopteran brain, because it harmonizes with 

 Heider's careful study of Hydrophihis {'89). He has failed to find 

 indications of segmental constrictions in the optic plate and 

 optic ganglion and his figure 4 A. B. at p. 37 agrees closely 

 with Viallanes' description of the Mantis brain. It should be 

 observed that the embryos of Hydrophihis are much larger 

 than those of Acilius and DorypJwra and therefore much more 

 favorable for surface study. On the other hand it may be 

 urged that Heider evidently did not employ so good a method 

 of surface preparation as Patten. 



The distinct invagination associated with the formation of 

 the optic ganglion in Coleoptera and described by Patten, 

 Heider and myself, is probably homologous, as Viallanes sug- 

 gests, with the intraganglionic thickening of the Orthop- 

 teran embryo. This structure in Mantis, and probably also 

 in XipJiidinm, takes no part in the formation of the optic 

 ganglion, which arises — at least in great part — by delamina- 

 tion as in the Crustacea (see Parker ('90)). Only the outer or 

 lateral portion of the optic plate becomes the compound eye, 

 so that in a later stage the intraganglionic thickening is separ- 

 ated from the edge of the eye by a considerable space. The 

 thickening then lies just laterad to the antennal furrow as 

 shown in Fig. 8. Whether or not the invagination in the 

 Coleoptera really plays any part in forming ganglionic tissue 

 as has been claimed, must be decided by renewed investi- 

 gations. 



