No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. ill 



The majority of authors hold that the Arthropod brain is 

 either wholly or in part homologous with the Annelid brain. 

 Patten ('90) alone takes the view that the Annelid prostomium 

 is absent in Arthropods, and that the brain of the latter is 

 formed by the moving forward of three segments which are 

 postoral in the Annelids. Apart from the fact that we have as 

 yet no means of deciding whether what we call the first seg- 

 ment of the Arthropod head (protocerebrum) is really a single 

 segment or a complex of several, it is extremely improbable 

 that so highly important a structure as the Annelid brain should 

 have completely disappeared in the Arthropods. So great is 

 the resemblance between the Arthropods and Annelids in all 

 the more important morphological features and even in the de- 

 tailed structure of the ventral nerve-cord, that the complete 

 elimination of the brain certainly makes strong demands on 

 one's credulity. 



Will ('88) goes to the opposite extreme and regards the 

 praeoral portion of the Arthropod brain as the exact homologue 

 of the Annelid brain. He goes so far as to call the procephalic 

 lobes of Aphis the " Scheitelplatte." He finds that they lie at 

 the pole of the &gg opposite the blastopore, or rather what 

 he takes to be the blastopore, and that they arise independently 

 of the nerve-cord. Now the "Scheitelplatte" oi Aphis must 

 include at least the proto- and deutocerebral segments — prob- 

 ably also the tritocerebrum. The deutocerebrum in all the 

 Orthoptera which I have examined is provided with a pair of 

 true mesodermic somites and with a pair of appendages, the 

 antennae. Each mesodermal somite sends a hollow diverticu- 

 lum into an antenna, which is thus shown to be homodynamous 

 with the other appendages of the embryo. The tritocerebral 

 segment also contains a pair of abortive somites and in Aniirida 

 maritiina, as I have lately ascertained, bears a pair of minute 

 but distinct appendages (see Fig. VI, tc. ap.). Viallanes ('87^) 

 and St. Remy ('90) have found that the second pair of antennae 

 in Crustacea belong to the tritocerebral segment. These facts 

 go to show that the deuto- and tritocerebral segments are homo- 

 dynamous with the postoral segments and, as the " Scheitel- 

 platte " of Aphis must extend at least as far back as the 



