5 L PROFESSOR E. RAY I-ANKESTEB. 



in the Vertebrate's raesoblast giving rise to the coeiom of 

 that group (pleuroperitoneal cavity) he regards not as the 

 primitive separation of the two deep layers (of ectoderm and 

 endoderra respectively) by which he supposes the coeiom to 

 have originated phylogenetically, but as a secondary splitting 

 following upon their heterotopous fusion. In this connection 

 it is important to observe that we have not much evidence 

 in favour of the view that the coeiom did phylogenetically 

 arise by the splitting of the two factors of the mesoblast, 

 that is to say, was a schizocoel (Huxley) . Possibly it was in 

 origin an outgrowth of the alimentary canal, a gastro- 

 vascular apparatus or enterocoel (Huxley), as its development 

 in Echinoderms, Terebratula, and Sagitta tends to prove. 



As the important general result of the two classes of ceno- 

 genetic phenomena, which Haeckel distinguishes, viz.. Hetero- 

 chrony and Heterotopy he points out that in the progress of 

 time those organs are more and more strikingly brought into 

 the foreground of an ontogeny which are especially charac- 

 teristic and important for the stem, class, or order to which 



suppressed. From one point of view, which is not here touched on by 

 Haeckel, the most important developmental phenomena are those of 

 Hypertrophy and Atrophy. In organic evolution (phylogeny) organs do not 

 arise de novo, but are produced by the moulding influence of gradually 

 increasing hypertrophy of this part and atrophy of that. In ontogeny such 

 hypertrophies or atrophies may present themselves in the course of the 

 developmental recapitulation in their due order, the order in which they 

 appeared in the series of ancestors. They then fall under the head of 

 ' palingenetic ' phenomena. But if they occur earlier or later than is tlieir 

 proper place they fall under the head of heterochronous cenogenesis. It is 

 clear then that cenogenesis in divisible into heterotopy, heterochronous 

 hypertrophy {positive gtOYiiV) and heterochronous atrophy (negative growth). 

 Thus the atrophy of organs which have made their appearance in the 

 embryo but disappear in the adult is very generally part of the palingenesis. 

 On the other hand, the total or very early suppression of organs in the 

 embryo such as the notochord absent from most Tunicate ontogenies, the 

 gill tufts absent from the ontogenies of Amnionate Vertebrata, which should 

 appear if the palingenesis were carried out though to disappear before the adult 

 form is attained, are examples of the heterochronous atrophy of cenogenesis. 

 It is to be noted with reference to the terms palingenesis and cenogenesis, 

 that they can only be used in relation to specified tracts of the organic 

 pedigree. The cenogenetic phenomena of an early ancestor of this or that 

 organism necessarily become part of the palingenesis of its descendants. For 

 instance, the development of the mesoblast from the hypoblast exclusively 

 may be as Haeckel supposes, a cenogenetic phenomenon in vertebrata 

 relatively to the ancestors of an earlier grade of development. Once, 

 however, acquired by the earliest Vertebrates, it becomes in other Vertebrates 

 relatively to them a palingenetic phenomenon. Thus in birds as represen- 

 tatives of the Vertebrata, we may speak of it as cenogenetic, but in birds 

 as compared with Vertebrates of the grade of development of the sharks, it 

 can only be spoken of as palingenetic. With regard to atrophy see my paper 

 on 'Development of the Cephalopoda,' this Journal, 1875, p. 46. 



