Morpliology and Phylogeny of Insects. AAl 



considers the same stmctures to be rudiments of the Troclio- 

 spliere. Will * suggests a new hypothesis, which at first 

 sight appears very simple and plausible ; for, while comparing 

 the internal germinal streak of Insects with the germinal 

 streak of Myriapods (which, as is well known, increases very 

 greatly in length, and finally bends together in the middle 

 and becomes invaginated into the nutritive yolk), he considers 

 the amnion of Insects to be homologous with the posterior 

 half of the Myriapod germinal streak. The Insects with an 

 internal germinal streak would consequently be phyloge- 

 netically older than those with an external one (contrary to 

 the opinion of P. Mayer, according to whom the reverse is 

 the case, and the Insects with an external germinal streak 

 are the older). Of all these hypotheses that which is pro- 

 posed by Tichomirow and Emery appears to me to be least 

 happy. The entire results of embryology and comparative 

 anatomy compel us to suppose that the Crustacea must have 

 had an origin separate from that of the rest of the Arthro- 

 poda ; so that a repetition of the Crustacean carapace in the 

 development of Insects appears to be simply impossible. 

 Will's hypothesis is hardly applicable to those Insects in 

 which the cephalic fold (which, according to Will, is a secon- 

 dary formation) of the amnion constitutes almost by itself the 

 entire amnion {Apis), while the caudal fold is very little 

 developed ; it is also very improbable that the cephalic and 

 caudal folds of the amnion, which are so similar in their 

 formation, were of quite different origin. Until the appear- 

 ance of Graber's paper f P. Mayer's hypothesis seemed to me 

 to be the most probable ; according to Graber, however, the 

 amnion in Melolontha consists not merely of ectoderm but also 

 of mesoderm, which is surely irreconcilable with the interpre- 

 tation of the amnion as a cast-off skin. Kennel's view, on 

 the contrary, appears to find confirmation in this remarkable 

 fact. Altogether the above-mentioned hypothesis of Kennel 

 seems to me to be the only one against which no evidence of 

 importance can be adduced. I therefore gladly allow with 

 Kennel that the embryonic envelopes are no new formation, 



* L. Will, " Entwicklungsgeschicbte der viviparen Apliiden," Zool. 

 Jahrb., Abth. f. Morpliol. Bd. iii., 1888, pp. 201-28(3, Taf. vi.-x. (Also 

 Arb. zool.-zoot. Inst. Wiirzb., Bd. 6, 1883, " Ueber die Embryonalent- 

 wicklung d. viv. Aphideu," Sitz.-Ber. naturf. f. Ges. liostock,"^ 24 mai, 

 1887; Arcb. Ver. Freund. Naturg. Mocklenb., 1887, 41 Jabrg. 1888; 

 " Zur Entwickluiigsgescb. d. vivip. Apbideii," Biol. Centralbl., viii,, 1888, 

 no. 5.) 



t Graber, " Vergl. Studien iiber die Keimbiillen und die Riickenbildung 

 der Insekten," 54 pp., 8 plates, 32 woodcuts, Uenkscbrifteu d. matb.- 

 naturw. Classe Kais. Akad. Wiss. Wien, 1888. 



