GENERAL CONSIDERATIONS. 289 



G-. General Considerations. 



We have seen above (Figs. 113, p. 226, and 114, p. 227) that, in 

 the Myriopoda, the germ-band, as it increases in length, is flexed 

 ventrally and sinks into the interior of the egg. In this invagination, 

 which Ave must imagine to have, come about at first through the 

 difficulty of accommodating the long germ-band within the spherical 

 egg, we shall have to seek (as Graber, Xo. 1-49, indicated, and Will, 

 Xo. 97, more recently proved in greater detail) the starting-point for 

 the development of the invaginated germ-band of the Libellulidae. 

 We shall therefore consider the invaginated form of germ-band as 

 the most primitive in the Insecta. A careful comparison between 

 the condition of the Myriopoda and that of the Libellulidae, indeed, 

 reveals certain differences. In the Myriopoda, the germ-band alone 

 is drawn into the invagination. In the Libellulidae, on the contrary, 

 in which the germ-band is comparatively short, it occupies only one 

 side of the depression, while the opposite side seems to be occupied 

 by a part of the blastoderm which has been drawn into the depres- 

 sion with the band, and is then known as the amnion. The part 

 of the blastoderm not concerned in the formation of the germ-band 

 in this case therefore is more extensive, and this marks the first 

 commencement of the formation of the embryonic envelopes. 



In the Myriopoda, the parts of the germ-band not drawn into 

 the depression remain simply uncovered. In the LAbellulidae, on 

 the contrary, they are grown over by a fold (amniotic fold) which 

 arises secondarily. This formation of folds is a new acquisition in 

 the Insecta by which the system of embryonic envelopes is com- 

 pleted. It is contrasted by Will (Xo. 97), as the secondary part 

 of the embryonic envelopes, to the primary part which arises by 

 invagination. We should, however, hesitate to lay much stress upon 

 this distinction. 



In the more highly developed and specialised Insect types, the 

 secondary formation of folds becomes more prominent, while the 

 development of the germ -band through invagination sinks into 

 the background. The germ -band grown over by the amnion is 

 thus derived from the invaginated band, and the development of 

 the former marks an ontogenetic advance, as the complicated process 

 of rotation is now eliminated. 



The cases of vestigial development of the embryonic envelopes 

 which have been observed in endoparasitic eggs {Pteromalina, 

 Taehinidae), in the eggs of the viviparous Cecidomyiidae, and 



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