THE INVAGIXATED AXD OVERGROWX GERM-BANDS. 



275 



C, D, k), does not take part iu the process of invagination. It remains on the 

 true ventral surface of the egg, and only becomes covered by the growth of the 

 embryonic envelopes (af) taking place in the usual manner. Thus the relation 

 of the anterior portion of the germ-band in this type answers to the description 

 given below for the second type. 



2. "When the formation of the germ-band is accompanied by the 

 growing over of an amniotic fold, the ventral plate, and the germ- 

 band which develops out of it retain throughout the course of 

 development the position which is typical of the later stages in 



Fig. 136. — Diagrammatic median sections, to illustrate the development of the Libellnlid egg 

 (after Brandt). A-C, development of the germ-band (A', A") accompanied by invagination. 

 D, development of the amniotic folds (af) which grow over the cephalic end of the germ- 

 band. E, the aperture of the amniotic cavity is closed, v, ventral side of the egg ; 

 d, dorsal side ; a, anterior, 6, posterior pole of the egg ; af, amniotic fold ; ah, amniotic 

 cavity ; a, amnion ; bl, blastoderm ; bp, ventral plate ; do, food-yolk ; k, cephalic end of the 

 germ-band ; 1;', anal end of the germ-band ; kh, germ-prominence or commencing invagina- 

 tion ; s, serosa. 



all Insect eggs. This type of development which is exemplified in 

 the Diptera (Chironornus, Simulia, Cecidomyia) is the one already 

 described (p. 268, etc.). The germ-band in this case essentially 

 belongs to the ventral side of the egg. Its anterior end corresponds 

 to the anterior pole of the egg, and its posterior end to the posterior 

 pole (if we do not take into account the dorsal extension mentioned 

 above, p. 271). There is therefore no reversal or rotation in this 

 case. The embryonic envelopes are formed by simple folds which 



