272 INSECTA. 



and posterior ends almost come into contact (Fig. 141, p. 283). The 

 germ-band thus appears at these early stages, i.e., roughly speaking, 

 during the first half of its embryonic development, dorsally flexed. 

 In the later stages, in consequence of the development of organs, and 

 the more complicated shape thus brought about, the band shortens 

 by contraction, so that finally the oral aperture comes to lie at the 

 anterior pole of the egg, and the anal aperture near the posterior 

 pole (Fig. 143, m and an). These positions of the apertures are very 

 typical of Insect embryos at later stages. The embryonic rudiment 

 now no longer appears curved dorsally, but is straight. Indeed, 

 curvature in an opposite direction often takes place, the most posterior 

 segment of the embryo appearing ventrally curved (Phryganeidae, 

 Lepidoptera, Hydrophilus, Blatta, etc., Fig. 142 C, p. 285, and Fig. 

 143 B, p. 286). 



Graber (No. 30) lias recently pointed out that the Insects may be divided 

 into two groups, according to the extension and the increase in the length of 

 the germ-band. In the first group the conditions of growth of the embryo 

 described above prevail, while, in the other forms (e.g., Blatta, Stcnobothrus) the 

 embryonic rudiment from the very first extends over only a quite short area of 

 the periphery of the egg, and during the whole of the later development never 

 grows in the same way as forms belonging to the first group. In forms in which 

 the germ-band is short, the dorsal curvature is naturally not noticeable in the 

 earlier stages, and the germ-band appears to be straight. The growth of the 

 embryonic rudiment in length also progresses more equally during the whole 

 development. Shortening is not perceptible in the later stages. Insects might 

 therefore be divided into two groups as having, on the one hand, germ-bands 

 which are long at first and shorten later, and, on the other hand, germ-bands 

 which remain from the first comparatively short. This distinction, however, 

 does not appear to us to be based upon differences of any importance. 



B. The distinction between the superficial and the 

 immersed Germ-band. 



The general description of the position and the origin of the 

 germ-band and the embryonic envelopes, given above (p. 268, etc.), 

 only holds good for some of the Insecta. These conditions are to be 

 found in many Orthoptera (Blatta), the Phnjganeidae, Lepidoptera, 

 Hymenoptera, many Diptera (Chironomus), and to some extent in 

 the Coleoptera. In details, however, an abundance of variations 

 which will be mentioned later are found to occur; these may be 

 traced back to the shape of the egg, the amount and distribution 

 of the food-yolk, and also to some extent to the vestigial condition 

 of the embryonic envelopes. In other groups of Insects (Pseudo- 

 neuroptera, Hemiptera), on the contrary, we find that the phenomena 

 manifested in the formation of the germ-band and the embryonic 



