484 



their position, and spread themselves more evenly over large 



parts of the postero- ventral part of the brain. From this 



region at least three pairs of nerves are given off to the mouth 



parts, so that this part of the brain may be said to constitute 



^ a distinct lobe — the oral lobe. 



(^ The nerve cells comprising the brain are of the same type 



'i as those of the ventral nerve cord; dendrites are present, 



' though usually very hard to see (fig. 224). The cells vary 



tfrom 3|/x to 7jul in diameter. 

 According to the investigations of Bauer (1904) the nerve 

 cells, and even individual ganglia, are developed from single 

 neuroblasts, which bud off daughter cells, which after dividing 

 once become transformed into nerve cells. In Nasonia this 

 does not appear to be the case. The single-celled neuroblast 

 stage is passed through in the very early embryo, and in the 

 larva of the first instar the various ganglia are already to be 

 seen as distinct accumulations of embryonic cells. 



THE VASCULAR SYSTEM. 



(a) The Blood. 



The blood is the great essential tissue which co-ordinates 

 the whole process of metamorphosis. It is the medium in 

 which the processes of destruction and regeneration occur; into 

 it the dying cells cast their products of degeneration, and upon 

 its substance the growing tissues nourish themselves. 



This has been made abundantly clear in the description 

 of the metamorphosis of the various organs; the actual 

 chemical changes, however, which go on in the blood cannot 

 be discussed here. It is sufficient to say that the globules and 

 granules into which the various larval organs degenerate are 

 to a large extent cast into the blood stream, where they 

 dissolve. The blood, in consequence, which is usually quite 

 "thin," becomes during the late hours of larval life, and the 

 early hours of pupal life, very "thick," and so heavily laden 

 with protein materials that it often stains very strongly in 

 preparations and appears as a structureless matrix in which 

 the other organs lie suspended. But as the imaginal organs 

 develop, these substances gradually disappear, and are no 

 longer visible a day after pupation. 



Frequently, however, the dead larval tissues do not have 

 time to dissolve in the blood stream; the leucocytes, instead, 

 assuming their important role of body scavengers, fall upon 

 the dead tissues and rapidly absorb them. This phagocytic 

 absorption of dead tissues is very clearly seen in the removal 

 of the salivary glands, of certain tracheoles (fig. 88), of the 

 temporary pupal midgut (fig. 153), of certain muscles (fig. 



