300 INSECTA. 



develop from the abdominal limb-rudiments present in the embryo. It should, 

 however, be mentioned that Haase (No. 153), following Uljaxix, has recently 

 opposed this view, although, as it appears to us. with insufficient reason, 

 maintaining that the gonapophyses should be regarded merely as secondarily- 

 acquired external appendages.* 



We cannot deny that a certain phylogenetic significance attaches 

 to the presence in the Insect embryo of abdominal limb-rudiments 

 that degenerate later. Considering the near relationship that exists 

 between the Insecta, the Myriopoda, and Peripatus, Ave must see 

 in the appearance of these rudiments the ontogenetic recapitulation 

 of the conditions belonging to an ancestral form of the Insecta, in 

 which all the body-segments were still provided with well-developed 

 pairs of limbs resembling the present thoracic limbs. We should 

 have to attach a certain importance to the fact that, in the Orthop- 

 tera, the embryonic limb-rudiments of the first abdominal segment 

 are always more developed than those of the following segments 

 and, in Mantis, exactly resemble legs. Since, in Campodea (Haase, 

 No. 153), a true rudiment of a leg is retained on this segment, we 

 are justified in raising the question Avhether, in the degeneration 

 of the abdominal extremities in the series of ancestors of the Insecta, 

 the hexapod condition was not preceded by an octopod condition. 

 This would explain the fact that the segment in question, in many 

 points of its development, resembles the thoracic rather than the 

 abdominal segments. 



The limb-rudiments which are found as sac-like bnlgings of the surface of 

 the germ -band are from the commencement of their development filled with 

 mesoderm. In most Insects there is at first no arrangement in the mesodermal 

 cells that enter the limb-rudiments, but the Orthoptera seem more nearly to 

 follow the Myriopoda and Pcripaius, in so far as, in them, diverticula of the 

 coelom extend into the rudiments (Cholodkowsky. No. 19 ; Graber, Nos. 26 

 and 30). 



D. Nervous System and Tracheal Invaginations. 



The rudiments of these two systems of organs help essentially to 

 determine the external form of the Insectan germ-band. The rudi- 

 ment of the nervous system usually appears very early, before the 

 limb-rudiments are recognisable. We find, as rudiments of the ventral 



* [Heymons (Nos. XVI. and XXII.) has investigated the development of the 

 cerci, gonapophyses, and stylets in Lcpisma and other Insects, and he concludes 

 that the cerci are true appendages, that the styles appear to be dermal processes 

 replacing true appendages and intimately related to them, and that the gonapo- 

 physes have no relation to appendages. Wheeler (No. XLI V.), on the other hand, 

 is strongly in favour of regarding the gonapophyses as modified abdominal 

 appendages. Uzel (No. XL.) regards the ventral stylets of Campodea as direct 

 derivatives of abdominal appendages. — En.] 



