136 THE EMBRYOLOGY OF THE HONEY BEE 



observed in a single cross section. Such a section, through the 

 mandibular segment at Stage XI-XII, is represented in figure 

 50A. Here the laterad extensions of the ganglia — composed of 

 ganglion cells — are seen in process of being directly transformed 

 into the lateral nerves innervating the mandibles. It will also 

 be noted that the clefts in the lateral cords, later filled by nerve 

 fibres, are in this section continuous with hollow spaces traversing 

 the centre of the lateral nerves, which spaces are also, at later 

 stages, filled by nerve fibres. Caudad of the gnathal region the 

 lateral nerves are much longer and relatively more attenuated, 

 and their development consequently more difficult to follow. 



The median cord, up to Stage XI-XII (Figs. 49A and B), has 

 altered little histologically from its condition at Stage VII, being 

 for the most part still composed of rather long prismatic cells 

 with large and clear nuclei. Its cells nevertheless are slowly in- 

 creasing in number by mitotic division and decreasing in size. 

 These divisions are equal, and appear rather inconstant in direc- 

 tion. In figure 50B a mitotic spindle is seen, directed obliquely. 

 The size of the nuclei of the intraganglionic portions of the me- 

 dian cord, after their severance from the hypodermis, continues 

 to decrease, while at the same time they lose their characteristic 

 elongated form, until at Stage XIV their appearance is the same 

 as that of the ganglion cells of the lateral cords, having undergone 

 a similar differentiation into functional ganglion cells. 



The intersegmental sections of the median cord, as already 

 stated, remain united to the hypodermis, except in the gnathal 

 segments and in the fifteenth, sixteenth, and seventeenth segments 

 (Fig. 51B). In these the median cord is taken up entire, the inter- 

 segmental portion being also severed from the hypodermis and 

 contributing to the formation of the ganglia of these segments, 

 which are fused to form compound ganglia prior to hatching. In 

 the gnathal region, however, a slight thickening of the ventral 

 hypodermis marks the boundary between the rudiments of the 

 mandibular and first maxillary ganglia. In the remainder of the 

 trunk region the intersegmental sections of the median cord separ- 

 ate from the rest and form processes of the hypodermis, whose 

 form, prior to hatching, is illustrated by figures 50C and E. At the 

 time of hatching (Stage XV) the larva elongates, involving a 



