52 PROFESSOU E. RAY LANKESTER. 



a series of phenomena of space-relations succeeding one 

 another in definite order, it admits of variation (and conse- 

 quently of appropriate discussion) in relation to the tAvo 

 elements of space and time. The perturbations in the palin- 

 genetic evolution of an organism which may be classed as 

 cenogenetic, are necessarily either dislocations of the palin- 

 genetic phenomena, • in so far as regards time or in so far 

 as regards space. Hence we may with Haeckel very con- 

 veniently speak of such phenomena as heterochronous or 

 heterotopous as the case may be. 



The appearance of an organ in an embryo at an earlier 

 period (that is to say, at a period of lower development of its 

 other organs) than that at which we have reason to believe 

 this organ made its appearance in the Phylogenesis or an- 

 cestral development of the form in question, is a frequent 

 example of heterochrony. As examples, Haeckel cites the 

 early appearance of the notochord, of brain and eyes, of the 

 gill-slits, and of the heart (before the vessels) in Vertebrata ; 

 the early development of the limbs, and of the segments (of 

 the primitive stripe) in Arthropods generally ; of the tracheae 

 of the Tracheata, and the liver of the Crustacea. The Mollusca 

 furnish instances in their early developing otocysts, and, I 

 may add, in the early appearance of the ' shell-gland ' or 

 invagination of the mantle, the Gasteropods in their lingual 

 rasp, and the Echinoderms in their calcareous skeleton. 



Similarly we have examples of heterochrony which consist 

 not in the early but in the late appearance of inherited 

 ancestral traits of organisation. This kind of dislocation is 

 very generally seen in the late development of the sexual 

 glands in all Metazoa, and in the postponement of the com- 

 pletion of the alimentary canal, even though its rough 

 materials take up their position at the normal palingenetic 

 period. A striking example is, according to Haeckel, the 

 late formation of the septum of the auricles in the embryonic 

 heart of the higher Vertebrata, which is subsequent to that 

 of the septum of the ventricles. In the phylogeny of the 

 Vertebrata, on the contrary, it is clear that the two septa 

 originated in the reverse order, as the Dij^neusta, the 

 Amphibia, and the Reptiles demonstrate. 



Heterotopy is even a more important element of the 

 cenogenetic modification of the primary palingenetic pheno- 

 mena. I have myself insisted on the fundamental importance 

 of this common process in the comparison of the delaminate 

 and invaginate forms of Gastrula or Planula, and in the 

 comparison again of the epibolic and embolic forms of 

 invagination (see 'Ann. Nat. Hist.,' May, 1873, p. 328, 



