OLIGOENTOMATA AND APTILOTA 185 



to the inner surface of the chorion. At this time also the labrum gradu- 

 ally elongates to overhang the stomodaeal invagination. 



Blastokinesis occurs in the same manner as in Isotoma, a buckling in of 

 the thoracic region (Fig. 107) until the embryo lies \A'ith its dorsal side 

 turned outwardly (Fig, 108), the amnioserosa shrinking and thereby 

 crowding the yolk into the embryo. During this process the dorsal organ 

 diminishes in size but maintains its position near the posterior margin of 

 the head lobes. A further lengthening of the embryo causes the caudal 

 end to push past the left side or, less commonly, the right side of the head. 

 The embryo thereby acquires a somewhat spiral position in the egg. The 



F^^ 





Fig. 107. — Campodea. Blastokine- Fig. 108. — Campodea. Embryo after 



sis of the embryo. completion of blastokinesis. {am. ser) 



Amnioserosa. (do) Dorsal organ. 



mid-gut epithehum, according to Uzel, is not completed until after 

 emergence of the insect from the egg which takes place 14 days after egg 

 deposition. 



THYSANURA 



The Silverfish {Lepisma saccharina L.) 



The egg of Lepisma saccharina is elongate, oval, and about 1 mm. long, 

 with the micropyle at the anterior end of a firm, tough chorion. The 

 cleavage nuclei migrate to the periphery except for some yolk cells which 

 remain behind. The minute germ disk, as described by Heymons (1897), 

 develops in the ventroposterior part of the blastoderm. Posterior to the 

 middle of the germ disk on the yolk side, a small area of inner-layer cells 

 appears probably by migration from the germ disk. The cells of the 

 blastoderm are large and fiat; those of the germ disk small and closely 

 packed. The former, since they take no part in the formation of the 

 embryo but form an envelope, constitute the serosa. The germ disk now 

 sinks into the yolk (Fig. 109), the membranous part surrounding the disk 

 constituting the amnion. The amniotic cavity, unlike that of most 

 insects, is relatively large and remains open to the exterior at the amniotic 



