280 



IXSECTA. 



of the egg. Only after the secondary yolk has withdrawn into the primary 

 body-cavity, and the connective strand has been absorbed, does this aperture 

 or gap close (Fig. 139 B), and the invagination then assumes a shape exactly 

 recalling the corresponding stage in the LibclluUdac. As the rudiment of the 

 germ-band continues to grow, it develops a hook-like curvature (Fig. 139 C), 



Fig. 139. — Diagrammatic median sections through five stages in the development of the egg 

 of a viviparous Aphis (adapted from Will). The orientation agrees with that in Fig 136. 

 The genital rudiment is omitted. A, invagination of the germ-band (fc') and growing in 

 of the secondary yolk (sd). B, closing of the pore through which the secondary yolk was 

 taken in. C, hook-like flexure of the posterior end of the germ-band (k"). D, rise of the 

 amnion-folds («/). E, development of the cephalic serosa (s'). of, amniotic folds; ah, 

 amniotic cavity; am, amnion; do, remains of the primary food-yolk with its yolk-cells; 

 dz, yolk-cells; /, follicle-epithelium; k, cephalic end of the germ-band (cephalic lobe3) ; 

 k', posterior section of the germ-band ; k", posterior end of the germ-band bent in like 

 a hook; I, primary body -cavity; s, serosa; s', cephalic serosa; sd, secondary yolk; 

 x, point at which the secondary yolk forms. 



which may later become double (Witlaczil, No 98). Certain changes in 

 position also take place. The curved rudiment, which is at first symmetrical 

 with regard to the median plane, is soon too long to retain its position, and 

 certain deviations in a lateral direction occur. The outer aperture of the 



