475 



it. These changes are very profound, and the nervous system 

 undergoes as marked a metamorphosis as does any other 

 system of larval structuresT" — 



Tlie increase in size of the larval nerve cells is difficult 

 to estimate since most of their cytoplasm is contained in the 

 long nerve fibres. In the defaecating larva, however, the 

 part containing the nucleus has usually grown from a struc- 

 ture which in the first instar measured about 4/x in diameter 

 to one with a diameter of 4J/a to d/ul, and sometimes slightly 

 larger. Tlie real increase in the size of the cells may be 

 judged when the growth of the great nerve strands is taken 

 into account. In young larvae these measure usually some 

 6-10/x in thickness, while in the defaecating larva they have 

 grown to a thickness sometimes as much as 30/x. 



Towards the end of larval life these larval cells begin 

 to develop large nucleoli and show the typical signs of 

 degeneration. -s. 



In 'the nerve cord at about the time of defaecation the 

 large neuroblasts — purely imaginal structures corresponding 

 in every way with the other embryonic cells which lie dormant 

 during larval life — begin to divide by mitosis, and a consider- 

 able increase in the number of cells within the nerve cord 

 occurs (fig. 227). These cells nourish themselves, in part, 

 at any rate, at the expense of the degenerating larval cells, 

 these "being always recognized by their great nucleoli and 

 pale cytoplasm which is in process of rapid absorption by the 

 developing nerve cells. In the nerve cord the larval cells lie 

 scattered among the now far more numerous imaginal nerve 

 cells, and large masses of disintegrating cells are also often 

 to be observed (fig. 227). In the brain this is even better 

 seen. The developing nerve cells, it seems, then simply absorb 

 the dead larval cells, growing at their expense, and in the 

 larva some twelve hours after defaecation no trace of the old 

 larval cells remains. 



As the nucleated portion of the nerve cells has thus dis- 

 appeared, the long columnar nerve strand and the fibres which 

 form the peripheral nerves likewise disintegrate. But the 

 appearances of degenerating nerve fibres in these two regions 

 are quite different. 



Within the nerve cord the degeneration of the two nerve 

 strands is so intimately associated with the regeneration of 

 the nervous system that it is impossible, as a rule, tO' see> even 

 with the highest magnifications, what is actually taking place. 

 Sometimes, however, and especially within the brain, this 

 may be seen, the larval nerve strands, as the nucleated portion 

 dies, begin to undergo a total disorganization, and in place of 

 the strands of most delicate, almost microscopically invisible 



u>^ 



