332 



IN SECT A. 



whole ventral surface of the ovum, and in many cases extends in its sub- 

 sequent growth not only over the whole ventral surface, but over a con- 

 siderable part of the apparent dorsal surface as well (Chironomus, Simulia, 

 Gryllotalpa, etc.). In Coleoptera, so far as is known, it commences as a less 

 extended thickening either of the central part (Donacia) or posterior part 

 (Hydrophilus) of the ventral surface, and gradually grows in both direc- 

 tions, passing over to the dorsal surface behind. 



Embryonic membranes. In the majority of Insects there are 

 developed enveloping membranes like those of Hydrophilus. 



The typical mode of formation of these membranes is represented 

 diagrammatically in fig. 181 A and B. A fold of the blastoderm arises 

 round the edge of the ventral plate. This fold, like the amniotic fold 

 of the higher Vertebrata, is formed of two limbs, an outer, the serous 

 membrane (se), and an inner, the true amnion (am). Both limbs ex- 

 tend so as to cover over the ventral plate, and finally meet and coalesce, 

 so that a double membrane is present over the ventral plate. At the 

 same time (fig. 181 B) the point where the fold originates is carried 

 dorsalwards by the 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 repre- 

 sented at the stage immediately preceding the closure of the dorsal 

 surface. 



By the time that 

 these changes are 

 effected, the serous 

 membrane and am- 

 nion are both very 

 thin and not easily 

 separable. The am- 

 nion appears to be 

 usually absorbed be- 

 fore hatching ; but 

 in hatching both 

 membranes, if pre- 

 sent, are either ab- 

 sorbed, or else rup- 

 tured and thrown 

 off. 



The above mode 

 of development of 

 the embryonic mem- 

 branes has been espe- 

 cially established by 

 the researches of Kow- 

 alevsky (No. 416) and 



FlG. 181. DIAGRAMMATIC LONGITUDINAL SECTIONS OF AN 

 INSECT EMBRYO AT TWO STAGES TO SHEW THE DEVELOPMENT 

 OF THE EMBRYONIC ENVELOPES. 



In A the amniotic folds have not quite met so as to 

 cover the ventral plate. The yolk is represented as divided 

 into yolk cells. In B the sides of the ventral plate have 

 extended so as nearly to complete the dorsal integument. 

 The mesenteron is represented as a closed sack tilled with 

 yolk cells, am. amnion; se. serous envelope; v.p. ven- 

 tral plate; d.i. dorsal integument; me. mesenterou; at. 

 stomodasum ; an i. proctodaeum. 



