INSECTS WITH INVAGINATED GERM-BAND. 



277 



exterior (Fig. 138 A). The thin layer of the blastoderm surrounding 

 the egg, which has retained its superficial position, now represent* 

 the serosa. 



In the subsequent stage, which is characterised by the possession 

 of limb -rudiments, the remarkable position of the germ -band can 

 be distinctly recognised (Fig. 138 A). We see that its cephalic end 

 (v) is directed towards the posterior pole of the egg, while its hook- 

 like posterior end (ab) is directed towards the anterior pole. We 

 can also see, from comparison with other stages (Fig 138 C), that 

 the ventral side of 

 the germ -band on 

 which the limb-rudi- 

 ments form is turned 

 to the dorsal side of 

 the egg. The defi- 

 nitive position of the 

 •embryo is brought 

 about by a process 

 of reversal or rota- 

 tion of the germ- 

 band, the embryo 

 undergoing rotation 

 round its transverse 

 axis, and being at 

 the same time evagi- 

 nated from the am- 

 niotic cavity (Fig. 

 138 B). This pro- 

 cess is commenced 

 by the fusion and 

 subsequent rupture of 

 the amnion and serosa near the cephalic region. This rent gives rise 

 to an opening into the amniotic cavity at the very point where the 

 original aperture of the invagination was situated, and through this 

 aperture first the head and then the consecutive segments of the 

 germ-band emerge and become applied to the ventral portion of the 

 egg-shell, the head shifting towards the anterior pole (Fig. 138 C). 

 In proportion as the embryo emerges from the amniotic cavity, the 

 latter diminishes in size, and finally completely disappears, the 

 •embryo being now only surrounded by the egg-shell. 



As the germ-band now lies on the surface of the egg, the area 



Fig. 137. — Three stages of development of the embryo of 

 Calopteryx (after Brandt, from Balfour's Text-book). The 

 embryo is represented inside the egg-shell, am, amnion ; 

 g, lateral edge of the ventral plate ; ps, rudiment of the 

 germ-band ; se, serosa. 



