396 INSECTA. 



of the Insecta, and shall feel inclined to regard the absence of the 

 facet- eye (whether in larvae or in imagines) as the result of 

 degeneration. 



We must, in conclusion, point out a few more important factors in 

 the embryonic development of the Insecta. The first of these is the 

 development of the embryonic envelopes, the acquisition of which 

 (like the development of flying) proves the Insecta to be the most 

 highly developed of all Arthropods. It therefore seems remarkable 

 that the Insecta in other respects, especially with regard to the way 

 in which the germ-layers form, have retained very primitive char- 

 acters. The long blastopore which extends over the whole of the 

 ventral side, the presence of a distinct invagination-gastrula which 

 leads to the development of an archenteric tube, and the manner in 

 which the mesoderm separates from the entoderm must here be 

 mentioned in this connection. "With regard to the last point, it 

 should be mentioned that the separation of the mesoderm from the 

 entoderm is accomplished by a process which can be traced back to 

 that of infolding, so that Kowalevsky (Xo. 49) quite correctly 

 compared the formation of the germ-layers in the Insecta with their 

 formation in Sar/itta, a proceeding in which he was afterwards 

 supported by Rabl. The coelomic sacs of the Insecta may thus, 

 according to their development, be regarded as archenteric diverticula. 

 Another point of interest is the transformation undergone in later 

 stages by the primitive segments, which were treated in detail in the 

 chapter on the development of the heart and the genital organs. 



LITERATURE. 

 Embryonic Development. 

 Older treatises by Herold, Kollicker, Leuckart, Rathke, 

 Robin, Suckow, and Zaddach. 



1. Ayers, H. On the development of Oecanthus niveus and its 



parasite Teleas. Mem. Boston Soc. Nat Hist. Vol. iii. 

 1884. 



2. Balbiani, E. G. Sur la signification des cellules polaires des 



Insectes. Compt. Mew J. Acad. Sci. Paris. Tom. xcv. 1882. 



3. Balbiani, E. G. Contribution a l'etude de la formation des 



organes sexuels chez les Insectes. Recueil Zool. Suisse. 

 Tom. ii. 1885. 



4. Barrois, J. Developpement des Podurelles. Assoc. Franc. 



A cane. Sci. 7 e Sess. 1879. 



