116 Mr. P. A. Buxton on the 



word to which so many meanings have been assigned. By 

 it I mean a mass of nerve tissue arising from that preoral 

 part of the embryo which is not segmented and which bears 

 no appendages. It is the nervous element corresponding 

 to the acron of some embryologists, and it is not the serial 

 homologue of the deuterocerebrum and tritocerebrum 

 (mesocerebrum and metacerebrum of some writers on the 

 segmentation of the Arthropoda). The protocerebrum of 

 insects is, in fact, the syncerebrum of Heymons, unless it 

 contains some element not yet differentiated as belonging 

 to the preantennary ganglion. 



In this paper I propose as I have said to deal solely with 

 the protocerebrum. I give, however, the following brief 

 summary of the function and connections of the other two 

 supraoesophageal ganglia. The deuterocerebrum is the 

 ganghon of the antenna, to which it gives motor and sensory 

 nerves. The pair of ganglia forjning the deuterocerebrum 

 are united across the middle line above the oesophagus, and 

 lie before and below the protocerebral lobes. They are the 

 antennary or olfactory lobes of some authors. The deutero- 

 cerebrum gives rise to the paired sympathetic system, which 

 lies upon the lateral wall of the oesophagus on each side ; this 

 consists of two pairs of small ganglia with nerves which con- 

 nect them to each other, and, as is kpown in some insects, to 

 the median or tritocerebral sympathetic system. The pair 

 of ganglia composing the tritocerebrum lie on each side of 

 the anterior part of the oesophagus and are generally fused 

 above to the rest of the supraoesophageal brain. The lower 

 part of the tritocerebrum is the circumoesophageal connec- 

 tive or commissure. This ganglion suppHes the labrum, 

 but has no paired appendage connected with it in the 

 insects; a band of fibres, the tritocerebral bridge, passes 

 across from one side to the other beneath the oesophagus. 

 The tritocerebrum also gives rise to a pair of fuie nerves 

 which run forwards and inwards to the frontal ganglion, 

 which lies upon the upper surface of the anterior part of the 

 oesophagus. This is the largest ganglion of the sympathetic 

 system : from it a fine nerve runs forwards and another 

 backwards. This latter, the nervus recurrens of some 

 authors, connects the frontal ganglion with a short chain of 

 ganglia lying on the upper surface of the oesophagus, and 

 from this unpaired sympathetic system the stomodaeum 

 is innervated. 



The suboesophageal ganglia or nerve masses will not 



