No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 93 



points in which the Blattid and Acridian nerve-cord differ from 

 that of the Locustid are so insignificant that I need not burden 

 the reader with their enumeration. I will stop to mention only 

 two peculiarities in Blatta. Here I fail to detect the pale 

 spots in the "slipper" stage of the germ-band, and sections 

 show that the neuroblasts do not differentiate as early as 

 they do in XipJiidunn. They are, however, readily detected 

 in late stages, when they stand out with even greater distinct- 

 ness than in the Locustid. The median cord neuroblasts, 

 though present and occupying positions corresponding to their 

 homologues in XipJiidiiun, are more dif^cult to trace, probably 

 on account of the smaller size of the embryo. 



Neuroblasts, or cells of a similar character have been de- 

 scribed and figured by a number of investigators of Arthropod 

 development. Perhaps the earliest mention of these cells is 

 to be found in Reichenbach's beautiful Astaciis monograph 

 ('86), where the nerve-cord is described as consisting in an 

 early stage of two kinds of cells — a few large pale elements 

 arranged in a single layer and confined to the periphery, and 

 a much greater number of small and more deeply stainable 

 cells forming the bulk of the ganglia. The developing ganglia 

 of the cray-fish resemble the ganglia of the Orthoptera in many 

 particulars. The number of large cells in the lateral cords in 

 Reichenbach's figures {notably his figures 1 14-133) is 3-6, the 

 average being 4 or 5, the same as in Xiphidium, Blatta, etc. 

 Furthermore the ganglia of Astacus show a foliated arrange- 

 ment of the smaller cells, which is not unlike the condition 

 seen in the older ganglia of the Orthoptera. Some of the 

 figures (188 and 189 for example) show a single neuroblast- 

 like cell surmounting the median cord cell-mass. There are, 

 however, two points in Reichenbach's work, which throw some 

 doubt on the homology of his large cells with the neuroblasts 

 of the Orthoptera. First, Reichenbach neither figures nor 

 describes these cells as dividing to form ganglion cells. This 

 negative observation, however, loses much of its force when 

 we consider that caryokinetic figures are singularly absent 

 from all of Reichenbach's figures, excepting his surface views 

 of young embryos, and when we recall the fact that amitotic 



