478 



u/ 1 1 be only one process by. which this takes place, viz., by the 

 amoeboid movement of the nerve cells through the fine 

 neuroglia network. It is evidently in this manner that the 

 cells move about within the nerve cord. 



In some ganglia the nerve cells may form layers five 

 cells in thickness; these gradually diminish in number at the 

 hinder and front parte of the ganglia and on the nerve strands 

 connecting adjacent ganglia may, at times, be quite absent. 



In the stomatogastric ganglion a destruction of larval 

 elements, followed by a development of imaginal nerve cells, 

 similar to that seen in the ventral nerve ganglia, occurs. It is 

 unnecessary to refer further to it here. 



It will be useful to point out that the apparent absence 

 of metamorphosis in the nervous system (except for the migra- 

 tion of ganglia), which is usually supposed to occur in insects, 

 has never yet been demonstrated. Even Weismann's observa- 

 tions on Corethra do not wholly disprove it, the destruction 

 of larval cells on a small scale being impossible to detect in 

 hand dissections. 



The most noteworthy feature of the metamorphosis of 



the ventral nerve cord is, then, the spontaneous degeneration 



of larval cells, and their destruction not by leucocytes, but 



j by a gradual process of absorption by the growing nerve cells. 



An average sized nerve cell from the imago measures 

 not more than 5/ji in diameter, though at times quite large 

 cells, as much as 12/x by 8/x may be seen. The cytoplasm is 

 usually much reduced, most of it having entered the nerve 

 fibre process. At times a small, or rarely very large, nucleolus 

 is seen. 



The splanchnopleural nerve sheath may be seen to be 

 continuous, at the termination of the nerves among the 

 organs, with the walls of the cells on which the nerve ends 

 (fig. 226). 



The Brain. 



While it will often be possible in the following descrip- 

 tion to refer to the nerve tracts within the brain, it is mani- 

 festly beyond the scope of this paper to make any attempt 

 to elucidate the actual nerve connections. 



The brain of the newly hatched larva (figs. 1, 230) is a 

 very prominent structure in the form of two large hemi- 

 spheres occupying the greater part of the second head seg- 

 ment, and projecting forwards into the first. It measures 

 about *15 mm. from side to side in its broadest region, and 

 is connected with the first ventral ganglion by a pair of short, 

 thick, circumoesophageal connectives, which pass backwards 



