THE EMBRYOLOGY OF THE HONEY BEE 257 



of the three gnathal segments unite to form the suboesophageal 

 ganglion. The remainder, fourteen in number, become the gan- 

 glia of the ventral nerve cord. The cells of the ectoderm con- 

 stituting the primitive swellings and protocerebral lobes become 

 segregated into two layers, an outer layer of small cells, the 

 dermatoblasts, destined to form hypodermis only, and an inner 

 layer of large cells, destined to form nerve tissue, the neuroblasts. 

 These divide unequally and teleblastically, giving of centrad sev- 

 eral cells smaller than themselves. These last mentioned cells 

 divide equally, the products becoming differentiated to form 

 ganglion cells. The latter are therefore the granddaughter cells 

 of the neuroblasts. The optic ganglia are not however produced 

 by the agency of neuroblasts, but are formed as simple infoldings 

 of the ectoderm. Beginning at the anterior margin of the man- 

 dibular segment, and extending to the last segment of the trunk 

 is a narrow median strip of ectoderm, the median cord. In the 

 intrasegmental regions this contributes the central portions of 

 the ganglia, in the intersegmental regions, it constitutes a series 

 of thickenings of the hypodermis. The supraoesophageal com- 

 missure is formed at least in part from the median ectoderm, but 

 this could not be established in the case of the suboesophageal 

 commissure. The stomatogastric nervous system is formed from 

 three median evaginations of the dorsal stomodaeal wall. The 

 first of these furnishes a few cells contributing to the formation 

 of the labral nerve, the second produces the stomatogastric gan- 

 glion, and the third the pharangeal ganglia. An outer neurilemma 

 only is present in bee embryos. This is formed from cells having 

 the same origin as the ganglion cells, and which migrate to the 

 external surface of the brain and ventral cord. The so-called 

 "ganglia" or corpora allata are formed from invaginations of the 

 ectoderm between the bases of the rudiments of the mandibles and 

 first maxillae, and are closely associated with the invaginations 

 which produce the apodemes of the retractor muscles of the 

 mandibles. The invaginations producing the corpora allata soon 

 become solid masses of cells, lose their connection with the ex- 

 ternal ectoderm and migrate mesiad and dorsad, finally becoming 

 attached to the ventro-lateral angles of the coelomic sacs of the 

 antennal segment. Degenerating cells are found in the rudiments 





