126 THE EMBRYOLOGY OF THE HONEY BEE 



with small dark nuclei. The former will give rise to the nervous 

 system and are therefore termed neuroblasts (Fig. 47A, Nbl), 

 the latter form only the hypodermis and the structures associated 

 with it; these cells as therefore termed dermatoblasts (Dbl). A 

 little later, when the neural ridges ana neural groove make their 

 appearance, the neuroblasts become arranged in four or five rows 

 on either side of the mid-line, lying next to the inner surface 

 of the ectoderm. The neuroblasts next begin to bud off by mito- 

 tic division smaller and darker cells from their inner surface. 

 Fig. 47B). These constitute the ganglion cells (GngC). Each 

 neuroblast thus gives off by division a number of ganglion cells 

 which together form a colum placed at right angles to the outer 

 surface of the embryo, each column having at its base the parent 

 neuroblast. Besides these neuroblasts lying on either side of the 

 mid-line there is another set, the median cord neuroblasts, which 

 are situated in the mid-line, at the bottom of the neural groove 

 (Fig. 47B.) These neuroblasts are intersegmental in their arrange- 

 ment, one neuroblast being placed between every two segments. 

 Each median cord neuroblast gives rise to a heap of ganglion 

 cells lying on its inner surface. These latter become displaced 

 cephalad and form the posterior median portion of each ganglion. 

 The remainder of the ganglion is formed from the invaginated 

 ectoderm of the neural groove. From this portion, Wheeler be- 

 lieves, arise the cells which form the inner and outer neurolemma. 

 The ganglion cells are the functional nerve cells of the ganglia 

 and send off delicate processes which branch and thus form the 

 nerve fibres or fibrillar substance which constitutes the connectives 

 and the commissures. 



The two divisions of the nervous system, the brain and the 

 ventral nerve cord form a continuous whole from their inception, 

 but for convenience they will be treated separately, following the 

 usual custom. The development of the brain is slightly in ad- 

 vance of that of the ventral cord and, in the latter, following 

 the general rule among arthropods, development progresses from 

 the cephalic to the caudal end. In the bee, however, the de- 

 velopment of the ventral cord is almost simultaneous in all 

 of the segments, the anterior being but slightly in advance of the 

 posterior. 



