NO. 3 EMBRYOLOGY OF FLEAS KESSEL 5 1 



which secondarily move anteriorly and dorsally to unite with the 

 deutocerebrum and protocerebrum in the formation of the supra - 

 oesophageal ganglion or definitive brain which lies dorsad to the 

 oesophagus (pi. 12, figs. 87, 88). This ventral origin is evidenced by 

 the fact that the transverse commissures (fused so as to appear one) 

 pass below the oesophagus (pi. 12, fig. 86). Ventrally, they unite 

 with the connectives of the most anterior of the trunk ganglia and in 

 this way help to form the so-called circumoesophageal connectives. 

 This third pair of cephalic ganglia appears as paired swellings lying 

 immediately posterior to and below the deutocerebral lobes. These 

 swellings are the tritocerebral lobes and the segment in which they lie 

 is known accordingly as the tritocerebral segment. 



The fourth, fifth, and sixth pairs of ganglia belong to the gnatho- 

 cephalon which includes the mandibular, the maxillary, and the labial 

 segments (pi. 3, fig. 33). They are all typical ventral chain ganglia 

 in their origin, each pair being united by two prominent commissures 

 in the early stages. Although arising separately, these three pairs of 

 ganglia ultimately fuse to form the single suboesophageal ganglion 

 of the late embryo and larva (pi. 3, fig. 26; pi. 12, figs. 86, 87, 88). 



The seventh, eighth, and ninth pairs of ganglia are developed in 

 the three thoracic segments. Unlike the coalesced neuromeres of the 

 gnathocephalon, these preserve their identity and are distinct in the 

 larva (pi. 12, fig. 87). 



The 10 posterior pairs of ganglia belong to the abdominal region, 

 one pair originating in each segment thereof except the most caudal 

 one. During development there is a shortening of the ventral cord 

 and a resultant anterior movement of these ganglia so that they do 

 not all remain in the segments of their origin. The definitive number 

 of abdominal ganglia present in the late embryo is eight. The first of 

 these remains in the first segment, but moves to its anterior region. 

 The second finally extends somewhat over into the first segment. The 

 third ganglion has its final position partly in the second somite and 

 partly in the third. The fourth ganglion takes up a similar position 

 between the third and fourth segments. The fifth ganglion entirely 

 leaves the segment of its origin and in the late embryo lies in the 

 posterior region of the fourth segment, In a similar manner the 

 sixth ganglion of the abdomen migrates into the posterior part of the 

 fifth segment. The seventh ganglion is even less conservative, as it 

 moves into the middle portion of the sixth segment. The eighth defini- 

 tive ganglion of the abdomen is somewhat longer than the others of 

 this region and is really a composite structure formed by the coa- 



