160 THE EMBRYOLOGY OF THE HONEY BEE 



from the second the pharyngeal ganglion, and from the third the 

 nerves passing caudad from the pharyngeal ganglia. The first 

 evagination in Forficula therefore corresponds apparently to the 

 second in the honey bee, the second to the third, while the last 

 is wanting. 



D. Neurilemma 



The neurilemma, which at Stage XV constitutes a thin cellular 

 membrane covering the external surface of the nervous system 

 (Fig. 37A, B and C, Nlm), first becomes evident in the ventral 

 cord at Stage XI-XII. At this stage, it will be remembered that 

 a considerable shifting and rearrangement of the cells of the 

 lateral cord takes place as shown in figure 50A and B. A large 

 number of the daughter cells of the neuroblasts have, in the intra- 

 ganglionic regions, shifted laterad, while others appear on the 

 ventral surface of the lateral cords where they form a single 

 layer. A similar layer is already present at the dorsal surface. 

 From these superficial cells the neurilemma is evidently derived. 

 In figures 50A and B, on the ventral side of the lateral cords, 

 the neurilemma may be seen to be in process of formation, certain 

 of the superficial cells having already assumed a flattened form 

 (Nlm). The development of the neurilemma on the dorsal side 

 of the ventral cord is less readily observed, owing to the number 

 of small cells crowded together in this region, but it seems reason- 

 able to assume that its origin is the same here as on the ventral 

 surface. The cells of the neurilemma at later stages (XI-XV) 

 vary much in size, indicating division subsequent to their assump- 

 tion of a superficial position. 



In regard to the origin of the neurilemma in insects there has 

 been a considerable difference in opinion. Nusbaum (1883, 

 1886) and Korotneff (1885) traced its origin to wandering blood 

 cells. Wheeler (1893) believed that it arose from the intragan- 

 glionic sections of the median cord. Heymons (1895, p. 45) states 

 that in Forficula "The outer neurilemma apparently arises from 

 cells which during the segregation of the neuroblasts from the 

 dermatogenous layer were separated off from the latter." 



In the honey bee it seems highly improbable that the neuri- 

 lemma arises from blood cells, since the cells which are destined to 

 form the neurilemma are when first evident — at Stages XI-XII — 



