Protocerebrum of Micropteryx. 115 



The brain of the Arthropoda in its fullest development, 

 that is to say as exemplified in the brain of the embryo of 

 Scolopendra (Heymons), consists of the following parts : 

 an archicerebrmn, which is median, impaired and preoral : 

 three lobes on each side, the syncerebral lobes, the outer 

 two of which arise from a common rudiment ; these also 

 are preoral, and together with the archicerebrum form the 

 syncerebrum : the preantennary ganglion, or protocerebrum, 

 which is the ganglion of the first somite, or true segment ; 

 we believe that this was primitively postoral, but it is 

 preoral in all living Arthropoda : the deuterocerebrum or 

 antennary ganglion, and the tritocerebrum or premandi- 

 bular ganglion, which correspond respectively to the second 

 and third somites. It may be said at once that the deutero- 

 cerebrum and the tritocerebrum correspond in Heymons' 

 nomenclature to the organs which I shall subsequently 

 describe under those names. This is not, however, the case 

 with the protocerebrum, for that word has been used in a 

 great variety of senses. In the development of the insect 

 head that part of the central nervous system which entomo- 

 logists generally call the protocerebrum (Viallanes) is 

 developed from the archicerebrum and the syncerebral 

 lobes : we do not yet loiow which parts of the insect brain 

 correspond to which of these structures, except that the 

 outer syncerebral lobe gives rise to the optic lobe, and 

 Haller suggests that the mushroom body is formed from 

 the archicerebrum. The preantennary ganglion or proto- 

 cerebrum of Heymons is not found at all in the insect 

 head, and is to be carefully distinguished from that jiart 

 of the brain which is commonly called by that name. The 

 synonymy is further complicated because the preantennary 

 ganglion or protocerebrum of Heymons is the precerebrum 

 of Verhoeff, and the word " protocerebrum " has been 

 used in yet a third sense to denote the procerebrum of 

 Heymons, that is the syncerebrum and preantennary gang- 

 lion (protocerebrum) together. The word protocephalum 

 has been used by Holste, and perhaps by others, to denote 

 that part of the brain which is dorsal to the gut in the 

 insects : *. e. the syncerebrum of Heymons (the proto- 

 cerebrum of insect neurologists since the time of Viallanes), 

 with the deuterocerebrum, and the tritocerebrum. 



I shall continue to use the word protocerebrum in the 

 sense in which neurologists from the time of Viallanes have 

 always employed it, though I should be glad to avoid a 



