104 GEORGO ORIHAY SHINJI 



exactly agree. Wheeler, for example, found four in Xiphidium, 

 while Nelson stated that in the honey-bee the number of these 

 cells usually varied from three to six. In coccids there are six 

 on each side of the median groove. 



Again, opinions are at variance as to whether the ganglioblasts 

 become directly converted into the nerve fiber or into their 

 daughter cells which constitute the nerve cells of the larva. 

 Wheeler ('89) for Doryphora and Nelson ('15) for the honey-bee 

 claim that all the ganglioblasts undergo at least one division 

 before they become nerve-cells, while Wheeler for Forficula 

 ('95), Lecaillon ('98) and Hirscher for Chysomelidae, and Essche- 

 risch ('02) for Musca maintain that the ganglioblasts do not 

 divide. In the coccids, as stated, the ganglioblasts do not 

 undergo any division, but, later, some of them are transformed 

 into nerve fibers. 



The neurogenesis in the case of the brain is exactly the same 

 as in the ventral nerve cord. The differentiation of the ammion, 

 mesoderm, and ectoderm is shown in figure 105. The ectoderm 

 cells soon give rise to the neuroblasts and thus are differentiated 

 into the dermal and neural layers. The segmentation of the 

 brain into three regions, corresponding in number to the future 

 brain segments, is also clearly noticeable in this figure. 



Figure 110 is a corresponding longitudinal section of a some- 

 what older embryo. There are about five large spherical cells 

 or neuroblasts along the periphery of each of the brain segments, 

 as was the case with the formation of the ventral nerve cord. 



In Coccids, as in other insects, there is originally one ganglion 

 in each of the body segments, making three in the brain, three 

 in the oral region, three in the thorax, and ten in the abdomen, 

 or nineteen in all. This number of ganglia is best seen in the 

 specimens shortly after the completion of the revolution (figs, 

 53 and 118). Shortly after the union of the stomadeum invagi- 

 nation with the entoderm or the midgut, all of the abdominal 

 gangha, except the first, disappear, leaving only the longitudinal 

 nerve fibers behind. These slender longitudinal nerve fibers, it 

 may be added, run out from the posterior margin of the only 

 surviving abdominal ganglia and innervate the abdominal organs, 

 such, for example, as the ovaries, midgut, etc. (fig. 132). 



