CHAPTER XVI 



TIIK (1EKM-LAYKRS AND EVOLUTION 



IN his papers of 1866 and 1867 Kovvalevsky had remarked 

 upon the widespread occurrence of a certain type or funda- 

 mental plan of early embryonic development, characterised 

 by the formation, through invagination, of a two-layered sac, 

 whose cavity became the alimentary canal. This develop- 

 mental archetype was manifested in, for instance, Sagitfn?- 

 Rrtiin? Lyimuca? AstacHs? P/wronis? Asterias? Ascidia? 

 the Ctenophora? and Amphioxus? He noticed also that 

 the invagination-opening often became the definitive anus. 

 Further instances of this mode of development were later 

 observed by Metschnikoff" and by Kowalevsky s himself, but 

 it was left to Haeckel to generalise these observations and 

 build up from them his famous Gastraea theory. This was 

 first enunciated in his monograph of the calcareous sponges, 9 

 and worked out in detail in a series of papers published in 

 iS/4-76. 10 



1 Gegenbaur, Zcits.f. ii.'iss. ZooL, v., 1853. 



-' Rcmak, loc. <://., p. 183, pi. xii. 



! Lereboullet, Ann. Sci. nat. (4) xviii., pp. 1 18-9, 1862. 



I Lereboullet, in Remak, p. 183 f.n. 



' Kowalevsky, Mt'-m. Acad. Set. St Pctcrsbourq (Petrograd), (7), x. 

 and xi., 1866 and 1867. 



A. Agassi/, Contrib. Nat. Hist. United States, v., 1864. 



" A/t'm. Acad. Set. St Pctersbourg (Petrograd), (7), xiv., 1869. 



s " Embryolog. Studien an Wiirmern u. Arthropoden," JA'w. Ai'<ni. 

 Sci. St Pctersbourg (Petrograd), (7), xvi., 1870. 



II Die K'nl/csc/i'n.'iiiiime, 3 vols., Berlin, 1872. General chapters 

 translated in Ann. Afiitf. Nat. Hist. (4), xi., pp. 241-62, 421-30, 1873. 



"' "Die Gastraea-Theorie, die phylogenetische Classification des 

 Thierrcii hs und die Homologie der Keimblatter." Jcnaischc /.eilschrift, 

 viii., pp. 1-55, 1874. " Die Gastrula und die Eifurchung der Thiere," 



288 



