MYRIAPODA 397 



These depressions are termed by Heymons the "median brain pits" 

 and are analogous to the ganghon pits of the rump segments. Cells 

 from these pits which later will join the archicerebrum take part in the 

 formation of the dorsal cortex (lamina dorsalis cerebri) of the protocere- 

 brum. About the time that the median brain pits begin producing gan- 

 glion cells, another brain pit ("lateral brain pit") is formed laterad of 

 the first and in front of the preantennae. These will give rise to ganglion 

 cells for the frontal lobes and probably also for the small optic lobes which 

 lie on the lateral margins of the frontal lobes, the last mentioned but 

 feebly marked in Scolopendra. Brain pits are also developed laterally 

 on the preantennal, antennal, and intercalary segments. The ganglia- 

 anlagen originating from these three pairs of pits are serially connected 

 with the anlagen of the ganglia of the gnathal and rump segments. Even 

 when the brain-pit capsules have become detached from the epidermis, 

 they still continue to function in giving off ganglion cells. Later the 

 archicerebrum and the adjacent ganglionic masses fuse so that their 

 limits are no longer sharply marked. The ganglia of the preantennal 

 segment form the broad connection between procerebrum and the 

 deutocerebrum. Several nerves arise at the j unction of procerebrum and 

 deutocerebrum of which two, somewhat larger than the others, may be 

 the preantennal nerves. The ganglia of the antennal segment form the 

 large deutocerebral lobes, which originally were placed behind the pro- 

 cerebrum but later migrate forward. The gangha of the intercalary 

 segment form the halves of the tritocerebrum, lying ventrad of the 

 deutocerebrum and later merge w^th this ^^dthout sharply marked limits. 



Scolopendra as well as some related chilopods lack well-defined 

 transverse commissures. In fact the subesophageal commissure is com- 

 pletely wanting. In the brain there is a broad transverse neuropile 

 mass which bridges the esophagus, but isolated commissures are not well- 

 differentiated. 



The visceral, or stomatogastric, nervous system becomes apparent 

 shortly after flexure of the germ band. On the dorsal walls of the 

 stomodaeum at the base of the labrum, differentiation of ectodermal cells 

 takes place. These cells become free to form the frontal ganglion. 

 When the ganglia of the intercalary segment are crowded dorsad by the 

 developing head, these cells are pushed against and unite with the frontal 

 ganglion. It is then no longer possible to say what part belongs to the 

 tritocerebral lobes and what part to the frontal ganglion. The whole is 

 now a united mass (Fig. 354, fg) whose median section certainly is 

 derived from the frontal ganglion and which has been designated as the 

 stomatogastric bridge. This bridge lies just dorsad of the esophagus and 

 is closely united with the underside of the forebrain. Between bridge 

 and forebrain is a foramen (Fig. 354) through which the aorta (ao), two 



