2S8 



INSECTA. 



of the germ-band develops (Fig. 144, k'), bends round dorsally, and 

 sinks into the yolk. The most posterior part of the germ-band is 

 thus here immersed ; the anal end is directed forward and applied 

 to the dorsal side of the egg ; in short, it shows all the characters 

 of the invaginated germ-band (Fig. 144, 7c). 



The germ-band, in the Coleoptera, is thus originally bent round dorsally 

 over the posterior pole of the egg. The cephalic end of the germ-band accord- 

 ingly lies at first some distance from the anterior pole (Fig. 134 D, p. 270). 



It, however, gradually moves towards the 

 anterior pole (Fig. 134 E), while the pos- 

 terior end moves back from its dorsal 

 situation to the posterior pole. This 

 shifting causes the posterior invaginated 

 portion of the germ-band to be, as it were, 

 drawn out of the yolk, so that, finally, the 

 germ-band throughout its whole length is 

 superficial. This shifting of the germ- 

 band corresponds exactly to the process 

 of rotation. In Hydrophilus, however, the 

 rupture in the embryonic envelopes takes 

 place only at a later stage. 



Conditions like those just described for 

 Hydrojihilus are found in the other Coleop- 

 tera, as may be gathered with special dis- 

 tinctness from the observations of Gkaeer 

 (No. 30) on Lina and of "Wheeler (No. 

 95) on Dory2)hora. Here also the posterior 

 end of the germ-band is bent in dorsally 

 and sunk into the yolk. The principal 

 difference between these cases and that of 

 Hydrophilus is found in the fact that the 

 cephalic end of the band appears from the 

 first near the anterior pole of the egg, and 

 conserpiently the movement accompanying 

 rotation is not here to be observed. 



"We have already pointed out (p. 284) 

 that the posterior end of the germ-band 

 in the Diptera is sunk into the yolk, in 

 the same way if not to the same extent as 

 in the Coleoptera. "We have here also the 

 last indications of the formation of a germ- 

 band by invagination. The presence of 

 these vestigial conditions, and above all the condition of Hydrophilus (and 

 Oecanthus) seem to indicate that the formation of the germ-band by invagina- 

 tion is the primitive method in the group of Insecta, while the growth over 

 it of the amniotic fold represents a secondary condition (Will, No. 97)., The 

 movement of rotation which can be observed in Hydrophilus and Oecanthus 

 is only comprehensible on this assumption. 



Fig. 144.— Diagram of a median longi- 

 tuilinal section through a Hydrophilus 

 embryo in the stage depicted in Fig. 

 134 1), p. 270 (after Heider). of, 

 anterior amniotic fold ; af, posterior 

 amniotic fold; o.h, amniotic cavity; 

 am, amnion ; do, food-yolk ; k, the 

 segmented germ-band, which is 

 already trilaminar; h', posterior end 

 of the germ-band bent round dor- 

 sally and sunk into the yolk ; v, 

 ventral side of the egg; d, dorsal 

 side of the same. 



