68 WHEELER. [Vol. VIII. 



at once be referred to the axis of the mature embryo. When 

 the eggs of other insects are oriented in the same manner, it is 

 seen that the germ-band invariably arises on the ventral sur- 

 face of the yolk with its procephaleum directed towards the 

 cephalic, and its tail towards the caudal pole. No matter what 

 positions it may subsequently assume, it always returns to its 

 original position before hatching. Frequently the germ-band, 

 when newly formed, lies nearer the lower than the upper pole 

 {Calopteryx, CEcant/ms, Stagviomantis, Hydrop/iihis, Qtc). The 

 usual movements are very simple ; from a position of rest on 

 the ventral surface of the G.gg, the germ-band moves through 

 an arc till its body is completely inverted. Then it rests and 

 again passes back through the same arc to its original position 

 on the ventral yolk. These movements may be compared to 

 the single vibration of a pendulum. The ascending movement 

 I shall designate as anatrepsis, the descending as katatrepsis, 

 the intervening resting stage as the diapmisc. The general 

 term blastokincsis may be used to include all the oscillatory 

 movements of the germ-band. 



Inasmuch as the germ-bands in other Arthropods (Crustacea, 

 Myrioix)da, Arachnida, and Thysanura) exhibit no movements 

 comparable to those of the lower Pterygota, and since, more- 

 over, the insect germ-band is formed in exactly the same 

 manner as that of other Arthropods and ultimately returns to 

 its original position, no matter what oscillations may intervene, 

 it is safe to infer that blastokincsis has been acquired within 

 the Hexapod and probably even within the Pterygote group. 

 We may also infer from the intimate relations of envelope- 

 formation to blastokincsis in most forms, that both of these 

 processes arose at about the same time. 



No attempt has been made to account for the origin of blasto- 

 kincsis. It has occurred to me that it may be due to causes of 

 a purely physiological nature. The eggs of the primitive Pter- 

 ygota were, as I have attempted to show, provided with a con- 

 siderable amount of food yolk. Like their modern descendants 

 they were probably also invested with dense chitinous envelopes. 

 These must render the respiration of the embryo difficult as 

 compared with embryonic respiration in annelids, mollusks and 



