TRACHEATA. 



401 



which usually contains nuclei, and in the Poduridae is divided up 

 in the ordinary segmentation into distinct yolk cells. The first 

 definite structure formed is a thickening of the blastoderm, 

 which forms a ventral plate. 



The ventral plate is very differently situated in relation to the yolk in 

 different types. In most Diptera, Hymenoptera and (?) Neuroptera (Phry- 

 ganea) it forms from the first a thickening extending over nearly the 

 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 directions, 

 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 repre- 

 sented diagrammatically in fig. 181 A and B. A fold of the 

 blastoderm arises round the edge of the ventral plate. This 

 fold, like the am- 

 niotic fold of the 

 higher Vertebrata, 

 is formed of two 

 limbs, an outer, 

 the serous mem- 

 brane (se), and an 

 inner, the true am- 

 nion (am). Both 

 limbs extend so 

 as to cover over 

 the ventral plate, 

 and finally meet 

 and coalesce, so 

 thatadouble mem- 

 brane is present 

 over the ventral 

 plate. At the same 

 time (fig. 181 B) 

 the point where the fold originates is carried dorsalwards by the 

 B. II. 26 



Sf 



FIG. 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 filled with 

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

 tral plate ; d. i. dorsal integument ; me. mesenteron ; st. 

 stomodaeum ; an i. proctodaeum. 



