EPHEMERIDA, ODONATA, PLECOPTERA, ET AL. 203 



The ventral plate invaginates to form a small spheroid embryo with a 

 central amniotic cavity. The lower part of this spheroid develops into 

 the amnion. The extraembryonic cells remaining at the egg periphery 

 form the serosa, completely enclosing the egg contents. A small mid- 

 ventral clump of residual serosal cells (grumulus) remains associated with 

 the caudal end of the embryo during early growth as a point of attach- 

 ment. A thick serosal cuticle is secreted on the outside of the serosa 

 beneath the chorion. This is partly eroded before hatching occurs. At 

 first the embryo lies upon the ventral serosa. As development proceeds, 

 it arches upward in the middle and finally becomes fully immersed in the 

 yolk, extending across the diameter of the egg. Although the embryo 

 becomes fully immersed in the yolk, this is not accomplished tail fore- 

 most, as it is in the Odonata, but by dorsal arching of the embryo, 

 followed by withdrawal of head and tail from the surface. The process of 

 invagination does not involve the formation of the amnion; the latter 

 completely covers the ventral side of the embryo before immersion begins. 



Just before blastokinesis the serosa and amnion rupture in front of the 

 head, and the embryo slips out of the yolk and turns so as to lie on its 

 right (ver}'- rarely left) side with its venter against the side of the egg and 

 the yolk dorsally enclosed by the reflected amnion, thus again attaining a 

 superficial position but now extending along the circumference of the egg. 

 The degenerate serosa forms a "dorsal organ," which probably incor- 

 porates the amnion also, as dorsal growth of the body wall progresses; 

 both embryonic membranes are thus engulfed in the yolk and disintegrate. 

 The embryo becomes coiled in the egg with its legs in the center and 

 finally hatches with the aid of a cephalic egg tooth. 



The inner layer, apparently representing mesoderm only, arises dorso- 

 caudally in the spheroid embryo as a small group of median cells, by 

 dorsal displacement and perhaps proliferation of cells from the outer 

 (ectodermal) layer without the formation of a gastrular furrow. Further 

 additions from below in the stages immediately following, if they occur at 

 all, are probably limited to the basal protocephalic region ( = differentia- 

 tion center?). Shortly after blastokinesis, indications of gonads appear 

 segmentally in the abdominal portions of the longitudinal lateral strands 

 of the splanchnic mesoderm. 



The embryo elongates, and protocephalon and protocorm differ- 

 entiate. No primary divisions occur; segments first become demarcated 

 in the gnathal region, and others are added posteriorly as the embryo 

 lengthens, by successive differentiation from the unsegmented tailpiece. 

 Labral, antennal, and intercalary pairs of coelomic cavities appear in the 

 protocephalon, in addition to the 3 gnathal, 3 thoracic, and 10 abdominal 

 pairs of coelomic sacs; the mesoderm of the eleventh abdominal segment 

 shows no distinct cavities. In the abdomen the first (pleuropodia) and 



