50 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 98 



The above account agrees quite closely with the observations of 

 Hatschek (1877) who pioneered in the embryological study of the 

 nervous system of insects, and also corresponds to the writings of 

 most insect embryologists. No observations were made in the present 

 study on the origin of the delicate ganglionic covering, the neurilemma. 

 Wheeler (1893) for Xiphidium and Eastham (1930) for Pieris are 

 of the opinion that it is produced by cells of the middle cord. 



The ganglionic swellings remain evident on the surface until about 

 the end of the fourth day of development. Their disappearance is 

 gradual and is due to the sinking of the ganglia to a lower level (pi. 3, 

 fig. 26; pi. 12, fig. 87). This sinking also serves to obliterate the 

 neural groove. 



The central nervous system of a flea embryo consists of 19 pairs 

 of ganglia. Figure 87 (pi. 12) shows the complete nerve chain, the 

 thoracic and abdominal ganglia having retained their identity, while 

 the cephalic ones are coalesced as described below. The first two of 

 these go to form the greater part of the definitive brain or supra- 

 oesophageal ganglion. The anterior pair, which at first forms the 

 independent protocerebral lobes, soon unites to form the bilobed 

 protocerebrum of the ocular segment. An apparently single com- 

 missural mass connects the two halves of this neuromere. The com- 

 ponent parts of the second pair of cephalic ganglia likewise fuse to 

 form a bilobed neuromere which in this case is known as the deuto- 

 cerebrum. It has as its function the innervation of the antennae. It 

 also has a single commissure. These first two pairs of ganglia are 

 the only ones of the entire nerve chain which have their origins 

 anterior to the position of the stomodaeal invagination. Although 

 the lateral cords continue anteriorly to this region, the proliferation 

 of the ganglion cells of the protocerebrum and deutocerebrum is diffi- 

 cult to follow. In this area the proliferation is very irregular. The 

 origin of the commissures of these neuromeres was not determined 

 definitely, but no indication of a middle cord was evident anterior to 

 the stomodaeum. This last observation is in agreement with the 

 account of Schaefer (1938) for Phormia. It seems, therefore, that 

 the connecting cell masses do not have an independent origin in these 

 cases but are produced merely by the approximation and fusion of 

 their respective pairs of ganglia. 



The brain of the flea embryo, like those of other insects, includes a 

 third pair of ganglia, which has a different origin from those of the 

 first two segments. This pair is derived from the lateral cords just 

 posterior to the stomodaeal invagination. Because of this origin, there- 

 fore, the elements of this pair are to be regarded as ventral ganglia 



