OLIGOENTOMATA AND APTILOTA 



183 



The periplasm is exceedingly thin. By the time there are 64 cleavage 

 nuclei, each in an island of cytoplasm, they reach the surface where 

 the nuclei continue to divide to form the blastoderm. At this stage no 

 nuclei remain behind in the yolk. At one side of the egg a small circular 

 area is formed where the blastoderm is thicker and provided with two 



Fig. 102. — Campodea. Section of egg with entoderm cells (e?ii). 



irregular layers of nuclei (Fig. 100). From all parts of the surface, nuclei 

 migrate toward the margin of the thicker area, with the result that the 

 upper two-thirds of the blastoderm loses most of its nuclei and the lower 

 third gains them (Fig. 101). Nuclei then leave the center of the thick- 

 ened area and pile up on the sides, leaving but a single layer in the center 

 (Fig. 102). Occasional cells come to lie above on the inner side of the 



ent— ^* 



ent-^ 



X 



\ 



I iG 103 — Campodea. Sec tion ot 

 egg with mesodeiin tells (mes) (ent) 

 EntoJoiin. 



^r-am.ser 



Fig. 104. — Campodea. Section of 



(aw. ser) Amnioserosa. (eci) Ectoderm. 

 (ent) Entoderm, (mes) Mesoderm. 



thickened ring of cells. These are the future entoderm cells (Fig. 102, 

 ent). Another migration of nuclei then takes place, reducing the thick- 

 ness of the ring wall and restoring nuclei to the upper part of the egg 

 as well as to the center of the ring (Fig. 103). The cells that increase in 

 number in the center of the ring later give rise to the mesoderm (mes). 



