396 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



24 pairs of ganglia, the first belonging to the maxilliped segment, 21 to the 

 rump segments, the last 2 to the pregenital and genital segments. The 

 last two pairs of ganglia later fuse in one. Ganglia are not developed 

 in the telson. 



The supra-esophageal ganglion, or brain, is here considered as com- 

 posed of three parts: the forebrain, or procerebrum (or protocerebrum in 

 the broader sense); the midbrain, or deutocerebrum ; and the hindbrain, 

 or tritocerebrum. The procerebrum again may be divided into a preoral 

 part which includes the archicerebrum, the lamina dorsalis cerebri, the 

 frontal lobes, and the optic lobes ; and a postoral part which includes the 

 protocerebrum (in the strict sense) and belongs to the preantennal segment. 

 The three ganglia of which the subesophageal ganglionic mass is composed 

 develop in the same way as those of the rump segments, except that they 

 form a compact body. 



pp arch ep 



^••nv/vJ;'* •♦'• '"'''*•'•*•'*' '^^\,y''''-~' 



\jC mes 



Fig. 353. — Scolopendra cingulata. Cross section of preoral part of germ band, {arch) 

 Archicerebrum. (ep) Epidermis, {mhp) Median brain pit. (mes) Mesoderm, (pp) 

 Proliferation pit. {yc) Yolk cell. {Adapted from Heymons.) 



The development of the brain is more complicated than that of the 

 subesophageal ganglion, four distinct anlagen entering into its composi- 

 tion. These are (1) an unpaired preoral anlage in the acron, the archi- 

 cerebrum; (2) two pairs of preoral anlagen, the lamina dorsalis cerebri 

 and the frontal lobes including the optic lobes; (3) a metameric pair of 

 postoral ganglia in the preantennal segment, in the antennal segment, 

 and in the intercalary segment (protocerebrum, deutocerebrum, and trito- 

 cerebrum) ; and (4) an unpaired preoral part of the visceral nervous 

 system, the frontal ganglion. To the statement that the preantennal and 

 antennal ganglia represent postoral somites Snodgrass (1938) offers the 

 objection that it is not substantiated by external evidence of segmentation 

 in the corresponding cephalic region of the embryo. The median archi- 

 cerebrum (arch) is formed first, before there is any indication of the 

 segmental ganglionic thickenings. Its anlage is formed in and restricted 

 to the clypeal region, developing from ectodermal cells by delamination 

 (Fig. 353, arch). Before delamination takes place, a shallow invagination 

 (Fig. 353, mbp), or depression, is formed on each side of the archi- 

 cerebrum, mesad and distinctly in front of the base of the preantennae. 



