132 THE EMBRYOLOGY OF THE HONEY BEE 



The separation between the lateral cords and the hypodermal 

 (dermatogenic) layer is more evident than before, being marked 

 in many sections by irregular clefts (Fig. 49C). The cells of the 

 hypodermal layer are still diminishing in size and now begin to 

 form an evident and well denned epithelium. The cell nests of the 

 lateral cords on the other hand, are becoming compacted together 

 to form the definitive ganglia. This is particularly noticeable in 

 the gnathal region (Fig. 50A) and at the anterior and posterior 

 ends of the thoracic segments (Fig. 49D). 



In addition to the mitotic figures of the neuroblasts it is not 

 unusual at this stage to find other mitotic figures within the cell 

 nests of the lateral cords. Two of these mitotic figures are to 

 be seen in the section illustrated by figure 49D. Both the size of 

 these mitotic spindles and their relation to the adjacent cells make 

 it clear that they belong to the daughter cells of the neuroblasts. 

 It is not clear whether all of the daughter cells thus undergo 

 mitotic division but it is at least certain that many of them do. 

 Wheeler, in his earlier paper on the histogenesis of the nervous 

 system of the grasshopper (1891), believed that the daughter cells 

 of the neuroblasts did undergo division. This opinion was re- 

 linquished in his later paper (1893) on the same subject, as 

 regards the grasshopper, but not as regards Doryphora, in which 

 form, Wheeler expressly states, the daughter cells of the neuro- 

 blasts divide. On the other hand Heymons (1895) for Forficula 

 and Orthoptera, Lecaillon (1898) and Hirschler (1909), for the 

 chrysomelid beetles, and Escherisch (1902) for Musca, deny the 

 division of the neuroblast daughter cells. 



At the stage next illustrated, Stage XI-XII, the entire ventral 

 cord — with the exception of its extreme posterior end — becomes 

 completely severed from the ectoderm and the ganglia begin to 

 assume their final shape. The separation of the lateral cords 

 from the overlying hypodermis was already evident at Stage X, 

 but at that stage the median cord was still embedded in the 

 hypodermis, the distal ends of the component cells of the cord 

 forming the bottom of the neural groove. These cells have mean- 

 while been increasing slowly by mitotic division. These divisions 

 are equal, no evidences of median cord neuroblasts having been 

 observed. Meanwhile the median cord has also separated from 



