SEGMENTATION OF HEAD. 321 



The two paired thickenings are so disposed that they continue on either 

 side the line of the paired ganglia of the neuromeres belonging to the 

 postoral part of the body, so that the arrangement raises the question 

 whether we have not in these medial and lateral paired rudiments the 

 representatives of segments anterior to the preantennal. This latter 

 however is the first in relation with which coelomic sacs and appendages 

 are formed, and we may, provisionally at least, reckon it, with Heymons, 

 the first metamere. 



With the complex syncerebrum the neuromere of the pre- 

 antennal segment (the protocerebrum s. str. of Heymons *) 

 becomes fused to form the dorsal lobes of the adult brain, the 

 procerebrum of Heymons, from which the optic nerves arise, while 

 the neuromere of the antennae forms the anterior paired lobes 

 giving off the antennary nerves the mesocerebrum or deuto- 

 cerebrum. The neuromere of the premandibular ganglion 

 forms the posterior and ventral pair of lobes the metacerebrum 

 or tritocerebrum, from which the commissures pass to the sub- 

 oesophageal ganglion. The suboesophageal ganglion is formed 

 by the union of three neuromeres, namely those of the mandi- 

 bular and two maxillary segments of Scolopendra. 



In the lower Insects (Apterygota, Orthoptera) the segments 

 of the head are laid down in a very similar manner, but no 

 trace has been found of the coelomic sacks or the appendages 

 of the preantennary segment. The antennal segment is here 

 the first to bear an appendage and to contain distinct meso- 

 blastic sacks. The premandibular segment is also well 

 developed, and in Forficula the nuclei of the mesoblastic 

 somite belonging to it are arranged in two layers, though 

 an actual cavity has not been recognized. The only cases in. 

 which appendages are known to be borne by this segment are 

 those of the primitive Thysanuran genus Campodea, in which 

 Hansen has found a pair of small tubercles representing them 

 in the adult ; and the Collembolan Anurida, in which Wheeler 

 and others have seen them for a short embryonic period. 



The supraoesophageal ganglion of Insects is likewise developed 

 from three paired masses (Wheeler, Viallanes), the procerebrum, 

 mesocerebrum and metacerebrum. Of these the two latter arise 



' Heymons uses the word protocerebrum, sensu stricto, for the 

 neuromere of the preantennary segment. The mass consisting of this 

 neuromere combined with the syncerebrum he calls procerebrum, but 

 also " protocerebrum, sensu lato." It will be called procerebrum in this 

 work. 



z rn. Y 



