474 



granules (fig. 10). They are neuroblasts from which the adult 

 nervous system will later develop. Lying more internally are 

 the functional la-rval nerve cells, considerably smaller than 

 the neuroblasts, and measuring about 4/>t in diameter. Each 

 has a large nucleus, surrounded by a very minute quantity 

 of cytoplasmic material, all the rest of the cytoplasm being 

 found in the long nerve fibres. The nerve cells are themselves 

 held together by a network of neuroglia cells, usually difiicult 

 to distinguish from the ordinary nerve cells, but clearly visible 

 in the region between adjacent ganglia. 



The nerve fibres are collected in two cylindrical nerve 

 strands running along the length of the nerve cord and giving 

 off branches to form the peripheral nerves in the various 

 ganglia. The double nature of the nerve cord is thus clearly 

 recognizable. It is usually only with the greatest difficulty 

 that the individual nerve fibres can be seen, so minute and 

 compressed together are they. 



The cells of the stomatogastric ganglion are similar to 

 those of the ventral nerve cord. 



The Post-embryonic Development and M etamorphosis of 

 the Ventral Nerve Cord. 



The cells of the nerve cord, like those of all the other 

 specialized larval organs, do not proliferate, but merely grow 

 in size. 



Tlie splanchnopleural covering of the nerves and nerve 

 cord may first be considered. While the embryonic imaginal 

 cells do not undergo any visible changes during the larval 

 period the larval cells grow greatly in size, and at the end 

 of the feeding period show the usual signs of defeneration 

 (figs. 221, 222), i.e., their cytoplasm becomes granulated; the 

 nuclei are long and oval, and greatly hypertrophied, 

 measuring 17/x in length, and contain a few scattered granules. 

 The usual prominent nucleolus, so characteristic of the 

 senescent cells of Nasonia^ is present. 



But shortly before defaecation, the embryonic cells spring 

 into activity, and dividing mitoticaHy") (fig. 222) rapidly 

 absorb and replace the dying larval cells, so that several hours 

 later the whole of the mesodermal covering of the nerve cord 

 has been regenerated. Towards the end of pupal life some of 

 the cells of this splanchnopleure develop great nucleoli, but 

 beyond this no visible changes are to be detected during^ the 

 pupal period. There is, therefore, no discontinuity .in -the 

 splauchopleure during its metamorphosis. Moreover, as it 

 acts as a sac to enclose the nerve cells, there can be no dis- 

 continuity of the nervous system as a whole during its meta- 

 morphosis, whatever the changes that may be going on within 



