No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 95 



to in this description is taken from a stage corresponding to 

 my Fig. 28. Botli in tliis figure and in Fig. 61 he represents 

 four neuroblasts in one of the lateral cords. Korotneff seems 

 not to have seen the early stages of proliferation. 



In the developing nerve-cord of Doryphora I observed ('89, p. 

 366) that "the outer layer of cells continuous with the hypo- 

 dermis stands off somewhat from the ganglionic thickenings, 

 leaving a space which is in early stages occupied by several 

 large, clear, oval cells which divide rapidly by caryokinesis, and 

 might be called ganglioblasts, as the products of their divisions 

 reinforce the mass of ganglion cells." In my figures the polar- 

 axes of the neuroblast spindles lie parallel to the surface of the 

 ganglia. Re-examination of my preparations has convinced 

 me that this observation is essentially correct. I find also that 

 the newly-formed daughter-cells of the neuroblasts occasionally 

 divide caryokinetically and thus give rise to further generations 

 of daughter-cells. The daughter-cells are not budded forth in 

 regular rows, but very irregularly. I am not sure that I can 

 distinguish the median cord neuroblasts in Doryphora, though 

 I believe that I have detected homologous structures. In my 

 figure 72 I represented circular intersegmental patches in the 

 median line between the lateral cords. Closer examination 

 shows these to be clusters of cells of the same appearance and 

 dimensions as the lateral-cord neuroblasts. They are very 

 clearly brought out by Graber in his figures of Hydrophihis 

 (•89, Figs. 40, 41, and 43, PI. Ill) and are described as taking 

 part in the formation of the posterior gangliomere of each 

 o-ano-lion. I doubt whether the large cells constituting the 

 posterior gangliomere of Periplaneta in Miall and Denny's 

 Fig. 43 ('86) to which Graber refers, are to be regarded as the 

 equivalents of the median-cord clusters in Doryphora and 

 Hydrophihis. Periplaneta very probably has in each segment 

 only one median-cord neuroblast, which atrophies before the 

 close of embryonic life, and the large cells in Miall and 

 Denny's figure probably arise from the daughter-cells and are 

 therefore merely large ganglionic elements. 



Graber figures and describes ('89, p. 47. PI- X, Fig. 130) a 

 cross-section through an abdominal ganglion of a Melolontha 



