308 



INSECTA. 



the serosa (s) tears ventrally and contracts dorsalwards (Fig. 152 B), 

 where it forms a dorsal organ closely resembling that of the Orthop- 

 tera and Rhyncota, which finally degenerates and sinks into the 

 yolk (Fig. 152 C). The amnion at first remains unchanged. The 

 completion of the dorsal region is brought about by the continuous 

 approximation of the points of union between the amnion and the 

 ectoderm ; this produces a narrowing of the dorsal umbilical passage, 

 which finally becomes obliterated by the fusion of its walls. The 

 amnion then separates from the ectoderm, and up to the time of 

 hatching surrounds the embryo as a closed sac (Fig. 152 C). 



B C 



s 



~ am. 



SM. 



V CZ- 



Fio. 152.— Involution of the embryonic integuments in Chironomus (diagram after Graber). 

 am, amnion ; r, dorsal umbilicus ; s, serosa, which in B has retracted dorsally and in C has 

 been absorbed into the yolk. 



D. Involution accompanied by the amputation of both 

 Embryonic Envelopes. 



This type may be derived from the preceding type if we imagine 

 that the serosa is not ruptured nor in any way essentially modified. 

 The dorsal extension of the germ-band, carrying with it the points 

 of origin of the amniotic folds, causes a constriction of the dorsal 

 umbilical passage and, by the fusion of its margins, completes the 

 dorsal integument ; the two membranes are first connected by a solid 

 umbilical cord with the embryo and, when this breaks down, they 

 become separated from the embryo and from one another (Fig. 

 143 B, p. 286). As two completely closed sacs, one within the 



