EPHEMERIDA, ODONATA, PLECOPTERA, ET AL. 



193 



indicating that maturation and fertilization occur previous to that time 

 (Fig. 113A). A short time later, cleavage nuclei are on their way to the 



Fig. 114:.— Libellula. (am) Amnion, {bid) Blastoderm, {gb) Germ band, {hi) Head 



lobe, {y) Yolk. 



periphery (Fig, 1135). In an egg six hours old two nuclei and three 

 cytoplasmic masses appear, indicating 

 this to be probably the eight-cell stage. 



The nuclei, as they migrate from 

 the center of the egg toward the 

 periphery, are surrounded by small 

 amounts of cytoplasm. The yolk 

 mass seems to be closely packed so 

 that each nucleus is tightly wedged in 

 by blocks of yolk with cytoplasm 

 streaming out in the interstices 

 between blocks. When the nuclei 

 reach the surface and enter the cortical 

 layer, they are spaced far apart (Figs. 

 113C, 114A), and the cytoplasm 

 between them appears thread-like in 

 cross section (Fig. 116, pr). Figure 

 115 shows a section of the blastoderm 

 after the cleavage nuclei have multi- 

 plied for some time and the yolk in 

 the vicinity has been liquefied. 



After the blastoderm is completed, 

 a strip on the ventral side of the egg 

 thickens, forming the germ band, the 

 remainder of the blastoderm becom- 

 ing very thin. A depression then appears near the posterior end of the 



Fig. 115. — Libellula. Section of 

 blastoderm {bid), {ch) Chorion, {y) 

 Liquefied yolk. 



