NO. 3 EMBRYOLOGY OF FLEAS KESSEL 49 



region. The appearance of the neural ridges is due to the active pro- 

 liferation of specialized ectodermal cells below the surface layer. 

 These primary nerve cells are called neuroblasts (pi. 9, fig. 72). The 

 neuroblastic thickenings along the lengths of the neural ridges consti- 

 tute the so-called lateral cords. Neuroblasts are also proliferated 

 from the floor of the neural groove, and these form the less prominent 

 middle cord. 



With the segmentation of the germ band, all of its layers are 

 affected except the entoderm. By this process the lateral nerve cords 

 are metamerically constricted into segmental divisions. The superficial 

 layer of ectoderm over the neuroblasts gives rise to the epidermis ; 

 therefore its cells are called the dermatoblasts (pi. 9, fig. J 2). The 

 neuroblasts on the other hand proliferate the definitive nerve cells. 

 Those of the lateral cords are particularly active in the intrasegmental 

 regions where they give rise to the masses of nerve cells constituting 

 the ganglia. Because of the fact that the lateral cords are paired, two 

 ganglia are produced in each segment. The ganglia of successive 

 somites are joined by the less thickened interganglionic portions of the 

 lateral cords, the connectives. Figure 27 (pi. 3) shows the fused 

 paired ganglia and their connectives in horizontal section, and like- 

 wise indicates that the two ganglia of such a fused pair are trans- 

 versely connected by two commissural neuropile tracts. These com- 

 missures appear to be formed from ganglion cells proliferated by 

 the neuroblasts of the middle cord. The paired nature of the com- 

 missures of a fused ganglionic pair is shown clearly by the two neuro- 

 pile tracts in each such definitive ganglion (pi. 12, fig. 85). From its 

 first appearance, this middle cord is segmented into chainlike thick- 

 enings of neuroblasts which correspond in position to the future 

 ganglionic areas. They are strictly intrasegmental, therefore, so that 

 there are no median intersegmental connections produced between 

 the connectives, such as Wray (1937) describes for Calcndra. In this 

 respect the development of flea embryos corresponds to Schaefer's 

 (1938) observation on Plwrmia. The neuropile of fleas, or the central 

 mass of fibrous tissue which is evident in the nerve tracts, appears to 

 be composed of the attenuated ends of the ganglionic cells closely 

 packed together. 



Later in embryological development, as described above, the two 

 ganglia of each segment become approximated to fuse closely at the 

 midline with the commissures and thus form a composite definitive 

 ganglion. The connectives retain their individuality, however, thus 

 preserving intersegmental evidence of the bilateral origin of the 

 ventral nerve cord. 



