JOURNAL. OF ENTOMOLOGY AND ZOOLOGY 215 



nerve fibers showing clear. F'ig. 47 is a section through one side 

 of the optic lobe and brain of the same stage, fiber area clear, 

 cephalic side up. Figs. 36 to 39 are cross sections through ventral 

 ganglia of an embryo 2.5 mm. long, ventral side up. Fig. 40 and 

 Fig. 41 are through one-half of the brain and optic lobes of the last 

 stage; the cephalic side is up. Fig. 42 shows the general arrange- 

 ment of the brain and head ganglia, as shown in a median longi- 

 tudinal section, the dorsal side is up. Fig. 43, is a longitudinal sec- 

 tion of the abdominal ganglion of a 2.5 mm. embryo; the dorsal 

 side is up. Figs. 44, 45 and 46 are from an embryo of 3.5 mm. 

 length; 45 shows a section of one-half of the brain, cephalic side 

 up, 44 and 46 are sections of the abdominal ganglion, frontal sec- 

 tions, cephalic end up. 



At another time it may possible to follow the development of the 

 histological elements of the central nervous system in more detail. 

 What is given now is a mere outline, and does not take up the his- 

 tory of individual cells or cell groups as far as may be possible 

 when the structure of the adult nervous system is carefully studied. 

 In this paper there has been no attempt to follow the developments 

 which do not relate to the central nervous system. A general sum- 

 mary of the development of the brain and ganglia is given as 

 follows : 



1. At a very early period the outline of the optic area may be 

 recognized, although the cells which form the brain and optic lobes 

 may not be the ones which give rise to this appearance. Such indi- 

 cations of this area are shown in Figs. 1, 2, 3, and 4. 



2. The cephalic portion of the nervous system is then first indi- 

 cated, the brain and optic lobes and the region of the three naupliar 

 appendages being formed more or less /';/ situ from ectodermal cells. 



3. The meta-naupliar region of the embryo is formed from 

 regular rows of teloblast cells, such as may first be recognized in 

 a stage like Fig. 24. The ectoderm from these teloblast cells 

 forms the covering for all the lower region of the embryo and along 

 the middle line gives rise to the cells which form the central ganglia, 

 as shown in Fig. 26. At such a period, when the location of the 

 nervous area is possible, six or seven rows of cells in the central 

 region may be considered to cover the nerve ganglion area. 



