276 INSECTA. 



rise from the edges of the germ-band (Figs. 132 and 133, pp. 267 

 and 269). 



If we take into consideration the position of the germ-band at 

 the time of its formation, the two types here distinguished might 

 also be defined as a type with the germ-band inversely placed, and 

 one with the germ-band normally placed. It would be still simpler 

 to define them as types either with or without the reversal or rotation. 

 It might indeed be objected that, in the germ-band that is overgrown,, 

 changes of position have also occasionally been observed, and these 

 are often very difficult to distinguish from true reversal or rotation. 



In the order Coleoptera, we shall find forms in which the forma- 

 tion of the germ-band affords a direct transition from one of the 

 types above distinguished to the other. 



D. Insects with Invaginated Germ-band. 



Libellulidae. We shall first consider the egg of the Libellulidae 

 (A. Brandt, ]S~o. 7) as the best representative of this type of develop- 

 ment. This family, as we shall see below (p. 288), seems to exhibit 

 conditions which might be direct modifications of those found in 

 the Myriopoda, and which must therefore be regarded as the more- 

 primitive. 



In Calopteryx, the first rudiment of the germ-band is found in a 

 thickening of the blastoderm (ventral plate) lying in the posterior 

 ventral half of the egg. The most posterior portion of the germ-band 

 soon becomes pressed into the egg (Fig. 137 A, g). While this 

 invagination, which by many authors is called the germ-prominence > 

 continually deepens, it becomes directed forward and grows out 

 towards the anterior pole of the egg (Fig. 137 B and C). The lumen 

 of the invagination is the first rudiment of the amniotic cavity. 

 A difference in the thickness of the two walls of the invagination 

 is very soon perceptible. The dorsal wall which represents the 

 amnion (am) becomes gradually thinner and its cells flatten, while 

 the other wall thickens and represents the actual germ-band (ps). 

 Almost the whole of the germ-band is here invaginated into the egg, 

 its posterior end pointing forward. Only a small part of the band, 

 its primitive anterior end, retains for a time the superficial ventral 

 position of the original thickening of the blastoderm (Fig. 137 C) ; 

 this soon broadens out to form the cephalic lobes. This part now 

 becomes completely grown over by a circular amniotic fold derived 

 from the surrounding blastoderm. When this circular fold closes 

 over the cephalic lobes, the amniotic cavity is cut off from the 



