No. I.] CONTRIBUTION TO INSECT EMBRYOLOGY. 99 



non-tiervous, should take part in forming the fibres of that 

 structure. 



Regarding the origin of the neurilemmata in insects, there is 

 still considerable doubt. The inadequacy and inconsistency of 

 Nusbaum's observations on Blatta ('83) have been sufficiently 

 pointed out by Eisig ('8?) and Korotneff ('85). Nusbaum derived 

 the median cord (which, by the way, he did not recognize as the 

 median cord) from the entoderm, and compared it with the 

 vertebrate chorda. So far his observations and conclusions 

 were erroneous, but he derived the inner and outer neurilemma 

 from the cells of this "chorda" — an observation which agrees 

 essentially with my own. 



Korotneff's view that the neurilemmata arise from migrant 

 mesoderm cells has not been confirmed by recent writers, 

 who are inclined to derive these envelopes from the ectoderm 

 (Heider, '89 ; Graber, '90). Though I venture to say that my 

 own observations are somewhat more definite than those hith- 

 erto published, I cannot regard them as in any way final. 



2. The Brain. 



In the following account of the Xiphidium brain, I shall use 

 the nomenclature employed by Viallanes in his recent papers 

 ('90^ '90^), since his studies on the brain-development of 

 Mantis religiosa agree very closely with my own. Before 

 passing to a description of my sections I would refer the 

 reader to the diagrammatic figure (VII) which represents the 

 main points in the structure of the embryonic brain. Here it 

 is seen that the ventral nerve-cord bifurcates just in front of 

 the mandibular segment and passes on either side of the 

 mouth, where it forms two successive pairs of ganglia. The 

 posterior of these {tc^ is the tritocerebrnm, or third brain seg- 

 ment. Its two halves are unit.ed by the infracesophageal com- 

 missure, shown in the figure as a broad white band connecting 

 the Punktsubstanz-masses of the ganglia. The anterior pair 

 of swellings {dc) constitute the deiUocerebrnm, or second brain 

 segment. From this portion the antennae are innervated. 

 Further forward the deutocerebrum passes into a large paired 



