402 INSECTA. 



dorsal extension of the edges of the ventral plate, which give 

 rise to the dorsal integument (d.i). This process continues 

 till the whole dorsal surface is covered by the integument. 

 The amnion then separates from the dorsal integument, and the 

 embryo becomes enveloped in two membranes an inner, the 

 amnion, and an outer, the serous membrane. In fig. 181 B the 

 embryo is represented at the stage immediately preceding the 

 closure of the dorsal surface. 



By the time that these changes are effected, the serous 

 membrane and amnion are both very thin and not easily 

 separable. The amnion appears to be usually absorbed before 

 hatching; but in hatching both membranes, if present, are either 

 absorbed, or else ruptured and thrown off. 



The above mode of development of the embryonic membranes has been 

 especially established by the researches of Kowalevsky (No. 416) and Graber 

 (No. 412) for various Hymenoptera (Apis), Diptera (Chironomus), Lepido- 

 ptera and Coleoptera (Melolontha, Lino). 



Considerable variations in the development of the enveloping membranes 

 are known. 



When the fold which gives rise to the membranes is first formed, there 

 is, as is obvious in fig. 181 A, a perfectly free passage by which the yolk can 

 pass in between the amnion and serous membrane. Such a passage of the 

 yolk between the two membranes takes place posteriorly in Hydrophilus and 

 Donacia: in Lepidoptera the yolk passes in everywhere, so that in this form 

 the ventral plate becomes first of all imbedded in the yolk, and finally, on the 

 completion of the dorsal integument, the embryo is enclosed in a complete 

 envelope of yolk contained between the amnion and the serous membrane. 

 During the formation of the dorsal integument the external yolk sack com- 

 municates by a dorsally situated umbilical canal with the yolk cavity within 

 the body. On the rupture of the amnion the embryo is nourished at the 

 expense of the yolk contained in the external yolk sack. 



In the Hemiptera and the Libellulidae the ventral plate also becomes 

 imbedded in the yolk, but in a somewhat different fashion to the Lepido- 

 ptera, which more resembles on an exaggerated scale what takes place in 

 Hydrophilus. 



In the Libellulidae (Calopteryx) there is first of all formed (Brandt, No. 

 403) a small ventral and posterior thickening of the blastoderm (fig. 182 A). 

 The hinder part of this becomes infolded into the yolk as a projection (fig. 

 182 B), which consists of two laminae, an anterior and a posterior, continuous 

 at the apex of the invagination. The whole structure, which is completely 

 imbedded within the yolk, rapidly grows in length, and turns towards 

 the front end of the egg (fig. 182 C). Its anterior lamina remains thick and 

 gives rise to the ventral plate (ps), the posterior (am) on the other hand 



