EMBRYOLOGY OF COCCIDS 103 



Comparisons and measurements of the neuroblasts in different 

 stages of development show that they are true teloblastic cells. 

 They do not diminish in size, but resume their original size after 

 each of a series of cell divisions. Thus, in fact, there is no cell 

 intermediate in size between the neuroblasts and the ganglio- 

 blasts. A comparison of figures 103 and 107 indicates that the 

 development of the ventral nerve cord takes place from in front 

 backward, as was the casein the development of the appendages 

 already described. 



The abdominal ganglia, the number of which corresponds with 

 that of the segments, are noticeable in the embryo, a longitudinal 

 section of which is shown in figure 118. The nerve cord in the 

 figure is completely separated from the ectoderm of the ventral 

 body wall in the cephalic as well as in the thoracic segments, but 

 in a few of the last abdominal segments they are still connected. 

 In fact, the separation of the ventral nerve cord, like all phases 

 of differentiation, takes place from before backward. Each mass 

 of ganglionic cells which now constitutes a ganglion, contains 

 a mass of fiber-like 'Punkt Substance' of the Germans, the 

 nerve fibers. The nerve fibers are of two kinds, the longi- 

 tudinal and the transverse (fig. 120), the former running parallel 

 to the long axis of the body, connecting with those of adjoining 

 ganglia, while the latter run at right angles to the longitudinal 

 fibers. 



The formation of nerve fibers takes place much the same as 

 was described for Xiphidium by Wheeler ('93) and for the honey- 

 bee by Nelson ('15), namely, by the elongation and subsequent 

 transformation of the cytoplasm and the disappearance of the 

 nucleus of the ganglioblasts (fig. 116). 



The neurogenesis described above agrees, in general, with the 

 results obtained by WTieeler in Doryphora and Blatta ('89) and 

 for Xiphidium ('93), Heymons for Forficula ('59), Graber for 

 Melolontha ('90), Lecaillon for Chrysomelidae ('90), Nelson for 

 Aphis mellifera ('15), and several other investigators in the case 

 of many other insects. 



As to the number of neuroblasts which produce the ganglio- 

 blasts in each half of a body segment, no two investigators 



