DKVKI.I P: 



135 



the oesophagi] I ire three neuroni* protocerebrum whi h 



is to bear tin- compound CyCSj < 2) dcittoccrchrum. or anteim.d QeUTOI 



rcbrum. which bdm<^ t< ll nt whi h bean tin- nidi 



mcntary intercalary appendage^ -pokcn o! al><> I;, hind t lie n-^.pha 

 ^iis arc. .it most, four QeUTOmereS, namely and in order, mnndilnd.ir, 

 '.'ingntil (found only in Collrmbnla . ni>ixill>iry and l<ihi,il. 



. 202. Diagrammatic transverse sections to illustrate formation of dorsal wall in 

 L>-plinotarsa. a, amnion (breaking up in C); g, germ band; s, serosa. After 

 \Vir- i the Journal of Morphology. 



Then follow the three thoracic ganglia and ten or eleven abdominal 

 Lrandiu. The first three neuromeres always unite to form the brain, 

 and the next four (always three; but four in Collembola and perhaps 

 oilier insects), to form the subcesophageal ganglion. Compound 

 ganglia are frequently formed also in the thorax and abdomen by the 

 union of primitive ganglia. 



Tracheae. The tracheae begin as paired invaginations of the ecto- 

 derm ' Fig. 204, /); these simple pockets elongate and unite to form the 

 main lateral trunks, from 

 which arise the countless 

 brunches of the tracheal 

 in. 



Mesoderm. From the 

 inner layer which was derived 



from the germ band by gas- PIG. 203. Transverse section -of germ layers 



, . , . of Clytra. c, ccelo/n sac; n,' neuroblasts (primi- 



trillation (FlgS. 192-194) are tive nervous cells). After LECAILLON. 



formed the important germ 



s known as mesoderm and entodcrm. Most of the layer becomes 



lerm. and this splits on either side into chambers, or codom 



' ' i, a pair to each segment. In Orthoptera these 



orlom sacs are large and extend into the embryonic appendages, but in 



Coleoptera, Lepidoptera and Hymenoptera they are small. These sacs 



-hare in the formation of the definite body-cavity, though the last 



independently, from spaces that form between the yolk and the 



