138 THE EMBRYOLOGY OF THE HONEY BEE 



The first two of the three swellings each possess two evident 

 commissures, clearly demonstrating that these two swellings 

 represent distinct ganglia, the third swelling on the other 

 hand, shows no evidence of commissures. In favorable trans- 

 verse sections, however, evidences of a few commissural fibres 

 may be seen in this swelling also. The best evidence for its 

 title to be considered as representing a pair of ganglia is obtained 

 in sagittal sections of Stage XI-XII (Fig. 51 A). Flere the ter- 

 minal swelling, like the two preceding it, is plainly seen to be 

 derived from a distinct trunk segment, the seventeenth (the 

 eleventh of the trunk), which is marked off by a well defined 

 constriction. There are therefore three pairs of ganglia repre- 

 sented in the terminal swelling of the ventral cord, the last pair 

 being rudimentary. 



In the embryos of insects generally either sixteen or seventeen 

 pairs of ganglia are recognizable ; in the latter case the last pair 

 is more or less rudimentary. Among the Hymenoptera Grassi 

 (1884) in the honey bee found sixteen pairs; Graber (1890) in 

 Hylotoma found sixteen pairs and the evident rudiments of a 

 seventeenth pair; Carriere and Burger (1897) in the embryo of 

 Chalicodoma found the rudiments of seventeen pairs of ganglia. 

 The investigators last named state that the rudiment of the last 

 (seventeenth) pair of ganglia is somewhat shorter than the others. 

 In figure XLV of their paper is a representation of the posterior 

 end of an extremely young larva, showing clearly the last three 

 pairs of ganglia of the ventral cord. These are here seen to be 

 subequal in size, the seventeenth pair appearing to be nearly if 

 not quite as large as the other two pairs. 



The origin of the lateral cords in insects is fairly well estab- 

 lished. In every insect embryo thus far studied they arise, as 

 Hatschek (1877) discovered, from the inner layers of the primi- 

 tive swellings. Wheeler (1891, 1893) called attention to the 

 role played by the neuroblasts in the development of the lateral 

 cords. Subsequent investigators of the development of the 

 nervous system of insects have uniformly observed similar telo- 

 blastic cells whose behavior corresponds, with the exception of 

 minor differences, to the account given by Wheeler. 



At the end of the embryonic period the neuroblasts undergo 



