304 INSECTA. 



[In Leplsma, one of the Thysanura (Heymoxs, No. XVI), the germ-band 

 attains a ventral flexure, and is invaginated into the yolk, in a manner 

 suggestive of the Diplopoda (p. 229), at a very early period. Here, however, 

 a slight amnion forms, and by a narrowing of the cavity of invagination an 

 amniotic cavity arises ; the amniotic folds, with the serosa, which latter sur- 

 rounds the greater part of the egg, however, never unite, so that the amniotic 

 cavity is never closed, and rotation takes place without rupture of the 

 embryonic membranes. The germ-band, commencing at its anterior end, simply 

 emerges from the amniotic cavity through tire persistent amnion-pore. The 

 serosa contracts dorsally, becomes invaginated, and forms the dorsal organ or 

 sac, which then disintegrates. This condition is distinctly more primitive than 

 that seen in the Libellulidac, and recalls the condition of the germ-band in 

 the Myriopoda. In this connection an important and highly suggestive paper 

 by "Willey (No. XLV.) should be studied. Willey believes that the amniotic 

 cavity of insect embryos was originally a product of invagination of the germ- 

 band, and that this invagination was primarily derived from and associated 

 with a ventral flexure of the embryo. In this respect he differs from Heymoxs, 

 who considers that the dorsal flexure of the Chilopoda and Poduridae is primi- 

 tive, whereas Willey would rather regard the ventral flexure of Lcpisvia and 

 the Diplopoda in this light. Willey further regards the dorsal organ of the 

 Poduridae and the indusium of the Locustidue as vestiges of a trophoblast such 

 as occurs in Perijudns novae-britanniae (p. 216). — Ed.] 



A. Involution through the development of a continuous 

 dorsal amnion-serosa sac. 



In describing the development of the Libellulidae (Fig. 138 C, 

 p. 278) we saw that, after rotation had taken place, the embryonic 

 envelopes (the amnion and the serosa) which had grown together, 

 represent a membrane which envelops the dorsal yolk-sac (am + se). 

 The condition then somewhat resembles that seen in the Mtiscidae 

 after the flattening out of the amniotic fold. In this membrane, 

 the part yielded by the amnion is clearly distinguishable from that 

 yielded by the serosa, for while the serous portion has greatly 

 thickened by continuous contraction to form a dorsal plate* the 

 amnion has retained its character as a delicate flattened epithelium 

 (Fig. 140, and D, am, r, p. 281). 



The further fate of the embryonic envelopes in the Libellulidae 

 has not been observed. We can, however, complete our description 

 by reference to other forms which show the same type of develop- 

 ment. As development advances, the food-yolk becomes more and 

 more restricted to the interior of the embryo, or more strictly 

 speaking, of the developing enteron. The yolk-sac consequently 

 diminishes in size, and the absorption of the food-yolk into- the 

 enteron produces a collapse of the dorsal plate, this latter sinking 

 in and forming a thick-walled sac, the so-called dorsal tube (dorsal 

 organ, Fig. 148, r). The walls of this sac soon undergo disintegra- 



* The dorsal organ of the Podurid embryo seems to be quite peculiar in its 

 formation, and cannot be referred to the dorsal plate here mentioned, as is 

 shown by its early appearance (Lemoine, No. 15). 



