THE EMBRYOLOGY OF THE HONEY BEE 147 



Figure 54C taken from the protocerebrum shows three spindles, 

 the two larger being plainly those of neuroblasts, while the smaller 

 is that of a daughter cell of the first generation. 



A minor point in which the histogenesis of the brain differs 

 from that of the ventral cord is the greater irregularity of the 

 groups formed by the neuroblasts and their progeny. In the 

 ventral cord these form more or less regular rows, lying for the 

 most part in the transverse plane of the embryo, while in the 

 brain the different neuroblasts and their progeny often appear 

 to form a confused mass. This is especially true of the earlier 

 stages, and is at least in part to be ascribed to the varying planes, 

 with regard to the morphological long axis of the embryo, in which 

 the brain is intersected. In the later stages more or less regular 

 pyramidal groups with a neuroblast at their outer and larger ends 

 are frequently seen. Some of these are shown in figure 56. 



Like the neuroblasts of the ventral cord, those of the brain 

 persist until after the hatching of the young larva. Probably they 

 remain active much longer than this. Sections through the brain 

 of a larva about two days old show cells with large clear nuclei 

 situated about the periphery of the brain and having all the ap- 

 pearance of the embryonic neuroblasts, except that they are larger. 

 Two of these larval cells were observed undergoing an unequal 

 and tangential division precisely like that of the embryonic 

 neuroblasts. 



At Stage X, when the brain has nearly reached its ultimate 

 embryonic dimensions, it begins to separate from the hypodermis. 

 This takes place in the same way as in the ventral cord, and the 

 hypodermis has for the most part the same character, being thin 

 and made up of flat cells. The deutocerebrum and tritocerebrum 

 separate from the hypodermis first. The protocerebrum separates 

 from the hypodermis more slowly, the separation beginning at? 

 the mesial border of these lobes at Stage XI, and slowly progress- 

 ing laterad, being completed as Stage XII (Figs. 52 and 55). 



The nerve fibres make their appearance at Stage XI (Fig. 55), 

 appearing simultaneously in the three divisions of the brain. Their 

 mode of development is precisely the same as in the ganglia of the 

 ventral chain, the formation of the nerve fibres being preceded 

 by the appearance of clefts beneath the innermost row of cells of 



