lOO SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL.97 



directly from the ectoderm of the clypeal region of the cephalic lobes 

 (fig. 43 B, dp), and appears before any of the appendages except 

 the antennae are formed. The paired rudiments are groups of gan- 

 glionic cells proliferated from the inner ends of small ectodermal 

 pits {igpsgp). The first of these rudiments to be formed {2gp) 

 lie at the extremities of the median rudiment, and their cells become 

 closely associated with the latter to produce a cellular mass that be- 

 comes the lamina dorsalis of the definitive protocerebrum. Laterad 

 of these rudiments are formed a pair of pits {igp) that furnish 

 principally the cells of the definitive frontal lobes of the brain, and 

 later when the eyes appear give rise also apparently to the small optic 

 lobes. Following the lateral rudiments of the lamina dorsalis on each 

 side are formed in series three other cephalic pits, which generate 

 respectively the primitive ganglionic centers of the preantennae 

 (Sgp), of the antennae (4gp), and of the appendageless tritocerebral 

 somite (sgp). The two series of neurogenic pits are continued pos- 

 teriorly on the mandibular, the maxillary, and the leg somites. 



Heymons regards the median unpaired brain rudiment as the equiv- 

 alent of the annelid "archicerebrum," but it would seem rather to 

 correspond with the ganglion of the apical plate of the polychaete 

 larva. The two paired rudiments that combine with the median rudi- 

 ment to form the definitive protocerebrum he refers also to the 

 "acronal," or prostomial, part of the head, but the preantennal, 

 antennal, and tritocerebral rudiments he claims represent postoral 

 somites. The preantennal ganglia constitute at first a connection 

 between the protocerebrum and the deutocerebrum, but later they 

 merge so completely into the brain that their identity is lost in the 

 definitive brain structure. The deutocerebral lobes formed of the 

 antennal ganglia lie primarily behind the protocerebrum, but with 

 the forward migration of the antennae they become transposed to a 

 position anterior to the protocephalon and take on a conical form 

 with the antennal nerves issuing from their distal ends. The trito- 

 cerebral lobes lie beneath the deutocerebral lobes and are continuous 

 with the stomodaeal connectives. 



The claim of Heymons that the preantennal and antennal ganglia 

 represent postoral somites is not substantiated by any external evi- 

 dence of segmentation in the corresponding cephalic region of the 

 scolopendrid embryo, and as represented in Heymons' figure (fig. 

 43 B) these ganglia appear to be actually preoral in position. In 

 none of the arthropods do the true cerebral ganglia have postoral 

 commissures, but the preoral position of their commissures in the 

 brain mass, Heymons says, is to be explained ontogenetically by the 



