EPHEMERIDA, ODONATA, PLECOPTERA, ET AL. 



197 



has again reached the surface (Fig. 120A). The proctodaeum, although 

 not so apparent as the stomodaeum, can also be distinguished at this 

 time. While these invaginations are deepening, the ventral nerve cord 

 is developing along the midventral line {71c). The neural groove appears 

 by the fourth day (Fig. 121), and on each side of it a number of neuro- 

 blasts differentiate from the ectoderm. They undergo mitotic division, 

 giving off daughter cells which soon form vertical columns above the 

 neuroblasts, conspicuous at 120 hours. The neuropile appears soon after 

 this as clear areas that are segmentally arranged (Fig. 120A). 



coel 



\ 

 neur neurg 



Fig. 121.— Plathemis. Cross section of germ band, {am) Amnion, (bl) Blood cell. 

 {ch) Chorion, (coel) Coelomic cavity, {neur) Neuroblasts, {neurg) Neural groove. 

 {ser) Serosa, {ysp) Yolk spherule. 



As segmentation occurs accompanied by the development of the nerve 

 cord, the mesoderm becomes divided into two longitudinal strips, and 

 these in turn are divided into segmentally arranged masses of cells 

 separated from each other by the developing intersegmental grooves. As 

 these segmental mesodermal layers continue to grow in width and length, 

 they fold over laterally to form the coelomic sacs, one on each side of the 

 center line in each segment (Fig. 121, coel). At this time cells of the inner 

 layer are set free and take their places next to the yolk. They are con- 

 sidered to be the blood cells (bl). 



After blastokinesis, the yolk, still constituting most of the egg, is 

 formed into spherules, each apparently surrounded by a delicate mem- 

 brane and containing within, a yolk nucleus. The embryo is a slender 



