THE EMBRYOLOGY OF THE HONEY BEE 115 



twice as broad as long, with a slight median depression on its ven- 

 tral surface. From its anterior and posterior ends arise thick 

 connectives joining it to the ganglia of the adjacent segments. 

 Each ganglion also sends out laterally a large nerve trunk 

 (Fig. 3/A, Nv) which soon breaks up into smaller branches and 

 these are distributed to various parts of the internal organs and to 

 the body wall. 



Histologically each ganglion is composed of a mass of cells, 

 the ganglion cells, each of which sends off a slender nerve fibre. 

 These fibres unite in the upper half of each ganglion with the 

 nerve fibres from other ganglia to form bundles or strands (Fig. 

 37A). A large longitudinal strand traverses the lateral halves of 

 the ganglia and passes through the center of the connectives ; 

 these two strands thus serve to link together all of the ganglia of 

 the nervous system, including those of the brain (Fig. 376 and 

 D). Two other smaller strands, the transverse commissures 

 (Fig. 3/A and D, Com) pass close together side by side from one 

 side of each ganglion to the other thus joining its lateral halves. 

 The nerve fibres in a ganglion consequently form a figure like the 

 letter H, the cross bar of the H, however, being double (Fig. 

 3/C). Each ganglion is thus incompletely divided into five 

 regions ; two situated laterad of the longitudinal fibres, and three 

 in the mid-line. These Wheeler (1893) following Graber, has 

 named the lateral, the anterior, the central and the posterior gang- 

 liomeres. Other nerve fibres also pass out from each ganglion 

 laterally, forming the core of the lateral nerves (Fig. 37A). 



An exceedingly thin and delicate cellular layer, the neurilemma, 

 (Fig. 37, Nlm) covers the exterior of the ventral cord and also 

 of the brain. 



The brain of an insect (Fig. 38) is to be regarded as composed 

 essentially of three double (paired) ganglia: the protocerebrum 

 (iBr), the deutocerebrum (2Br), and the tritocerebrum (^Br), 

 corresponding respectively to the protocerebral, deutocerebral, and 

 tritocerebral segments. These brain divisions consist of pairs of 

 more or less evident swellings, each of which is made up of a 

 mass of ganglion cells traversed by a thick central core of nerve 

 fibres continuous with those of the longitudinal connectives of the 

 ventral nerve cord. The protocerebrum of the young bee larva 



