haeckel's recent additions to gasti(^a-theory. 51 



An Account 0/ Professor Haeckel's recent Additions 

 to the Gastr^a-theory. By Professor E. Ray Lan- 

 KESTER, M.A., F.R.S. With Plates VII, VIII, IX, X. 



In the last immbers of the ' Jenaische Zeitschrift ' Pro- 

 fessor Haeckel has continued his important discussion' of the 

 facts of development in connection with the significance 

 of the primitive germ-layers as the key to animal genealogy. 

 Some useful terms are introduced by him to designate 

 phenomena which have been more or less clearly recognised 

 during the dev^elopment of this subject in the past three 

 years, and further, some original observations on the develop- 

 ment of particular animals are given. By the courtesy of 

 Professor Haeckel I am able to place before the readers of 

 this Journal four of the instructive plates which illustrate 

 his recent contributions to the Gastrsea-theory ; and I pro- 

 pose to give some account of his views, without, however, 

 committing myself to complete agreement with them. 



Palingeny and Cenogeny. — These terms were introduced 

 by Haeckel in his Anthropogeny, and will no doubt be 

 found useful. In the ontogeny, or individual development of 

 living forms, we have been accustomed under the new regime 

 of Darwinian embryology to distinguish those forms and 

 structural dispositions occurring in the course of develop- 

 ment from the egg, which appear to be due to heredity, 

 from those which appear to be due to adaptation. 



It has been recognised^ that there are two distinct 

 tendencies to be estimated in the process of ontogeny : the 

 tendency to recapitulation of the complete series of ancestral 

 forms, the effects of which Haeckel terms Palingenesis, and 

 the tendency to adaptation to present conditions resulting in 

 the suppression and hurrying over of steps in the recapitula- 

 tion and the development of special larval or embryonic 

 organs. The effects of this tendency are what Haeckel sums 

 up as Cenogenesis. Cenogenetic phenomena have hitherto been 

 spoken of as ' falsifications ' of the recapitulation, as ' abbrevia- 

 tions ' and larval ' adaptations.' In the series of ontogenetic 

 phenomena presented by an organism, it is clear that it is 

 the palingenetic portion of them which have most value for 

 the morphologist, whilst to the physiologist the cenogenetic 

 are especially interesting. 



Heterochrony and Heterotopy. — Ontogeny being essentially 



1 See this Journal, 1874, for a translation of Haeckel's essay, "The 

 Gastrsea-theory," 



2 See " Primitive Cell-layers of the Embryo," ' Annals and Mag. Nat. 

 History,' May, 1873, p. 322. 



