FROM EGG TO INSECT — SNODGRASS 401 



In the insect embryo, after the mesodermal fri-oove has (dosed, there 

 appears another median ventral jj^roove alon<2; the entire lenpjth of 

 the germ band (fig. 17A, NIG), while a ridge {NIR) swells out on 

 each side of it. Large cells now separate internally from the ecto- 

 derm of the ridges and the groove, which are the primitive nerve 

 cells or neuroblasts {NBl). The neuroblasts multiply and form 

 strands of smaller nerve cells along the midline of the embryo be- 

 tAveen the ectoderm {Ect) and the mesoderm {i\Ieso), and these 

 strands of nerve tissue are the foundation of the central nervous 

 system of the insect. There is a lateral cord {LG) generated in each 

 ridge, and a median cord {MC) formed above the median groove. 

 Figure 17 A shows the relation of these various tissues in a diagram- 

 matic way; at B and C of the same figure, taken from the work of 

 J. A. Nelson on the honeybee, the developing nerve cells are shown 

 more as they actually appear in cross sections. 



Note that the position of the central nerve cord of insects (fig. 21, 

 YNC) is just the opposite of that of the nerve cord of vertebrates. 

 Are the nervous systems of these two groups of animals, therefore, 

 of independent origin, or do insects and other invertebrates with a 

 ventral nervous system, and vertebrates with a dorsal nervous sys- 

 tem carr}^ themselves in reversed attitudes? 



Almost from the beginning, the insect nervous system sliows a 

 segmentation corresponding with that of the body; indeed, the nerve 

 segments are often more conspicuous than the body segments. The 

 segmental nerve cell masses are known as ganglia., and the connective 

 parts as confimisures. Each segment at first contains a \)i\iv of 

 ganglia (fig. 18 A. Gng), and the two of each pair are connected 

 crosswise by a pair of connective strands [Con), besides being con- 

 nected w4th the preceding and following ganglia by the commisures 

 {Com). In the first head segment an optic ganglion {OpGng) is 

 formed on each side between the central ganglia and the ectodermal 

 rudiments of the eye {E). 



As the embryo develops, the simple and primitive condition of its 

 nervous system is more or less obscured and lost by a uniting and 

 condensing of some of the ganglia into larger and more complex 

 ganglionic masses. First, the central ganglia of the first three head 

 segments (the segments of the procephalon) and the optic ganglia 

 unite to form the brain of the adult (fig. 18 B, Br). Next, as the 

 segments bearing the mouth appendages become added to the liead, 

 the ganglia of these segments unite to form the second compound 

 ganglionic mass of the head {SmGng). When the anterior part of 

 the alimentary canal is formed (fig. 20 B, Stom) it grows inward 

 between the crosswise connectives of the second and tliird pairs of 

 brain ganglia. As a consequence the connectives of the third ganglia 



