480 



brain amongst the larval cells, towards the great nerve tract, 

 and seems to constitute the "bourrelet intraganglionaire" of 

 Viallanes. On the internal postero-dorsal portions of the 

 brain are two pairs of masses composed of rather small imaginal 

 neuroblasts; they are the anlagen of the four ocellar ganglia. 

 There is another cluster of imaginal cells lying one on the 

 anterior ventral part of each of the large hemispheres near 

 the larval ganglia and representing the cells from which the 

 antennal ganglia of the adult will later develop (fig. 230). 



During the period of active growth of the larva there is 

 the usual increase in size of the purely larval elements in 

 the absence of cell division. The neuroblasts do not, so far 

 as I can observe, undergo any division during this period. 

 But at the end of this period of activity (fourth day of larval 

 life) the metamorphosis commences, at first slowly, but a day 

 later (at the time of defaecation) with apparently much 

 greater rapidity. 

 f^ When the brain of the larva in the defaecation period 



J is examined in sections the cells which constituted the func- 

 I tional part of the brain during larval life are seen to be in a 

 I state of advanced degeneration (figs. 77, 78, 235). The cells 

 are small and highly granular; nuclei are visible often only 

 with difiiculty; large nucleoli are usually present. Some- 

 times the cell outlines are already becoming indistinct and 

 the whole mass is obviously in a state of active disintegration. 

 Leucocytes have not been able to penetrate to these cells, 

 4^ and histolysis is entirely of a non-phagocytic nature. The 



great nerve tracts also show obvious signs of degeneration at 

 this same time ; distinct fibrillation of the tracts gradually dis- 

 appears, and sometimes a faint indication of degeneration 

 into fatty and other globules becomes manifest. But visible 

 degeneration in the areas, whose structures in the living state 

 is difiicult enough to observe, is never so pronounced as in 

 the surrounding areas where the nerve "cells" are dying. 



Contemporary with this extensive cellular degeneration 

 a pronounced activity of the neuroblasts is to be observed. 

 In the defaecating larva the neuroblasts have already greatly 

 proliferated by mitosis, and active mitosis in various parts is" 

 still to be observed, especially in the anlagen of the optic 

 ganglia (fig. 235). And as these cells rapidly increase in 

 number they nourish themselves in part upon the dead masses 

 of nerve cells and nerve fibres, and growing gradually in bulk 

 in the larva eighteen hours after defaecation, absorb and 

 replace these altogether. The cells of the two pairs of ocellar 

 ganglionic anlagen proliferate rapidly. Similar changes occur 

 in the imaginal cells (neuroblasts) of the antennal ganglion. 

 In the larva at about the time of defaecation, two kinds of 



