FORMATION OF THE EMBRYONIC MEMBRANES 533 



primitive band and germ-layers, and which covers the outer surface 

 of the yolk. Thus the serosa, which is usually held to include this 

 portion also of the blastoderm, 

 forms a closed sac which covers 

 the whole surface of the egg, with 

 one part extending over the surface 

 of the yolk, and the other over the 

 primitive band (Fig. 510). 



The inner of the two layers, 

 called the amnion (Fig. 509, am), 

 is more closely connected with the 

 embryo. The amnion and ecto- 

 derm of the primitive band to- 

 gether form a completely closed 

 sac, whose lumen forms the 

 amniotic cavity. Originally con- 

 nected with the serous membrane, 

 it splits off from the primitive 

 baud about the time the appen- 

 dages begin to bud out, and con- 

 tinues to closely envelop the body 

 and appendages, as seen in Fig. 

 509. Both of these membranes 

 are, before the time of hatching, 

 either absorbed, or, as in Lepi- 

 doptera, retained. The amnion is 

 retained until after hatching in 

 the locust, etc. In certain Coleop- 

 tera the serosa is retained, and the 

 amnion is absorbed (Fig. 532), 

 while in Chironomus and the 

 Trichoptera the serosa is ab- 

 sorbed, and the amnion retained, 

 with the egg-shell or chorion. 



FIG. 509. Diagrammatic cross-section through 

 three successive stages of the primitive streak, and 

 growing embryonal membranes of insect-embryos. 

 A. formation of the ventral plate and of the gas- 

 trula invagination ((/). , upward growth of the 

 amnion-folds (/). C, complete overgrowth of 

 the primitive band through the amnion-folds : 

 v, ventral side ; d, dorsal side ; a/, amnion-folds ; 

 ah, amnion-cavity ; am, amnion; bl, blastoderm; 

 bp, ventral plate ; do, yolk ; ec, ectoderm ; s, 

 serosa ; it, under or inner layer. After Korschelt 

 and Heider. 



