1 66 THE EMBRYOLOGY OF THE HONEY BEE 



A considerable number of the degenerating cells are not re- 

 tained within the brain tissue ; as early as Stage X the debris 

 of degenerating cells, consisting of the shrunken nuclei and the 

 spherules just described, may be seen on the dorso-lateral surface 

 of the brain, and this debris remains here beneath the amnion 

 apparently unaltered up to the time of hatching, as figure 62 

 shows. Degenerating cells continue to be extruded from the brain 

 up to Stage XV and possibly even later. At Stages XIII and 

 XIV, when the maximum number of degenerating cells are found 

 in the brain, the characteristic debris of these cells is visible within 

 the space between the hypodermis and the brain in almost every 

 section, and is especially abundant over the dorso-lateral region 

 between the proto- and deutocerebrum, where degenerating cells 

 are most abundant (Fig. 62). 



At Stage XV, that is, just subsequent to the hatching of the 

 larva, the number of degenerating cells has greatly diminished, 

 these having either been expelled or absorbed by the brain. A 

 few, however, are still visible. The debris lying on the outside 

 of the brain, within the head capsule, has also disappeared, having 

 probably been washed away by the blood current. 



The significance of this extensive cell degeneration is not appar- 

 ent. That these cells are not artifacts is shown by the uniformity 

 with which they occur, being present in all series of sections 

 examined of embryos which were older than Stage IX. Their 

 presence may accordingly safely be considered as normal. Vial- 

 lanes (1891), Wheeler (1893) and Heymons (1895) state that 

 the neuroblasts, or at least the major part of them, degenerate at 

 the close of embryonic development — a kind of senile degeneration 

 of the cell. This, as already stated, is not the case in the honey 

 bee. Moreover the degenerating cells begin to appear at a period 

 when the neuroblasts are just beginning to be differentiated. The 

 possibility that some of the neuroblasts degenerate is however by 

 no means excluded. At Stages XIII and XIV, many of the 

 superficial cells on the dorsal side of the protocerebrum are in a 

 state of degeneration, but these are in most cases at least, peri- 

 pheread of the neuroblasts. This seems to indicate that these 

 degenerating cells are derivatives of the neuroblasts, which have 

 wandered peripherad of the latter, as do the neurilemma cells 

 (p. 160), but their precise origin is unknown. 



