On the Moi'phoJogy and Phijlogeny of liiftecfs. 429 



LV. — On the Morphology and Phyhgeny of Insects. 

 By N. ClIOLODKOWSKY *. 



" Comparative anatoiii)' will have to confine itself more and more to 

 tLe raising of problems, while tiie exact formulation and solution tliereof 

 is tlie province of embryolosry.'" — Klf.inenberg. 



Among the embryolog-ical plienomena which are of importance 

 for phjlogenetic deductions the segmentation of tlie germinal 

 streak certainly occitpies a prominent position. This will 

 tiiercfore be the a])])ropriate place for the discussion of the 

 question as to the number of the segments of the germinal 

 streak and of their paired appendages. I shall leave out of 

 the question the so-called primary segmentation observed by 

 Ayers, Graber, and Nusbaum — in the first place because it 

 has as yai been but very little investigated, and secondly 

 because I doubt that this primary segmentation was of great 

 phylogenetic importance. For it is quite possible that the 

 early division of the germinal streak into four sections is 

 occasioned by similar causes to those which are responsible 

 for the early a])pearance of bilateral symmetiy in Vertebrates 

 and Arthropods or of the shell in Mollusks, i. e. by reaction 

 of the definitive shape of the animal upon the form of the 

 embryo. It may be added that as long ago as 1870 Metsch- 

 nikow described a similar primary segmentation in Scorpio^ in 

 which the germinal streak at first divides into three large 

 sections. 



'J'he total number of the segments of the germinal streak 

 of Insects is slated by authors to be from sixteen to eighteen, 

 and is said to be at any rate not more than eighteen. The 

 foremost segment, ^vhich bears the antennae, is universally 

 considered to be pre-oral^ while the remaining segments are 

 stated to form the primary trunk; the first three of these 

 belong to the head, the fourth to the sixth body-segments to 

 the thorax, and the seventh to the seventeenth to the abdo- 

 men. The last (eleventh) abdominal segment is not con- 

 sidered to be entirely homologous with the other metameres, 

 and is termed the " end-segment." The above is the preva- 

 lent conception of the Insectan germinal streak at the present 

 time, and in accordance with this are also interpreted the 



* Translated from the ' ]Memoiresde rAcademielmpt^riale des Sciences 

 de St. Petertbouig,' vii* s^rie, t. xxxviii. no. 5, ])p. 80-101 (St. Petersburg, 

 1891) ; being the concluding portion of a memoir by the same author 

 entitled "Die Embryonalentwicl<lung von VhyUodroDiia (lihdta) f/er- 

 vumku " {il>id. jip. l-li*0, with six plates). 



