118 COMPARATIVE ANATOMY chap. 



G. von Koch. MonograpTiie cler Gorgoniden. In : Fauna unci Flora des Golfes von 

 Mapel. 1887. 



R. Hertwig. Die Aktinien der Ohallenger Fxpedition, Jena, 1882. 



E. Haeckel. JReport on the Siphonopho7-a of the Challenger Fxpedition. 1888. 



A. Goette. Ueber die Fntwickehmg von Aurelia aurita und Cotylorhiza tuberculata. 

 1887. 



Numerous important works and treatises by Metschnikoff, F. E. Sohulze, Haeckel, 

 Moseley, L. Agassiz, A. Agassiz, Glaus, Weismann, Hamann, Grenacher, 

 Laoaze-Duthiers, Jourdan, Jikeli, Lendenfeld, Cliun, C. Keller, Wilson, Fol, 

 Semper, Dana, G. von Koch (a series of important treatises on Corals in 

 Gegcnbaur's Morph. Jahrh., vols, iv-ix), A. von Heider, etc. 



The fundamental law of Biogenesis. — Egg segmentation and the development of 

 the two primary germinal layers of the Metazoa (gastrulation) — The ontogeny 

 of the Cnidaria. 



Every Metazoon is, at the commencement of individual existence, a simple cell, 

 an egg cell ; i.e. its development starts from a point at which the Protozoon remains 

 during its whole life. Further, by the repeated division of the fertilised egg of every 

 Metazoon, a germ is produced, whose structure repeats in a general way the structure 

 of a simple Ccelenterate. This germ, which is known as the gastrula, consists of two 

 cell layers, the ectoderm and the endoderm, which may be compared with the two 

 layers composing the body of a simple adult Cixlenterate. 



Fundamental law of Biogenesis. — The frequently observed parallelism between 

 tlie consecutive stages of individual, or ontogenetic development, and the gi'ades of 

 development presented by the animal kingdom, is thus explained by the theory of 

 descent. Every animal, in its ontogeny, passes through, in an extraordinarily ab- 

 breviated and concise manner, the long series of its ancestral forms. " Ontogeny, 

 or the history of the development of the individual, is a short recapitulation of 

 the history of the race, or phylogeny." This sentence, in which the fundamental 

 law of Biogenesis is formulated, contains a generalisation of the fact that every 

 animal passes on to its descendants by inheritance, not only its organisation at an 

 adult stage, but also its own course of development. 



In the course of time adaptation, i.e. the survival of the fittest in the struggle for 

 existence, interferes with the action of heredity, so that species do not remain 

 constant, but change according to circumstances. In the same way, the ontogenetic 

 process of development also, i.e. the series of consecutive stages of development of a 

 species, may be subject to such modification that it no longer faithfully recapitulates 

 the process of development of its ancestors. The repetition of ancestral development 

 caused by inheritance is called " palingenetic " ; the modifications of ancestral 

 development caused by adaptation are " caenogenetic. " 



It is extremely diflieult to determine in a concrete case what is palingenetic and 

 what is csenogenetio. It is at once evident that a purely palingenetic course of 

 development never occurs anywhere. It is only by the discovery of similarity be- 

 tween one stage of development in an animal and another adult animal, that we can 

 decide that that stage has something palingenetic in it. "When a comparison of a 

 stage of development in one species with other species in an adult condition is not 

 possible, we have no sure means of judging what is palingenetic in it and what 

 CEenogenetic. For instance, we know of no adult animal with which the Eohinoderm 

 larva can be compared ; and we cannot consequently know whether these larvte have 

 in any way retained the organisation of their primitive Echinoderm ancestors. 



Our justification, again, for holding that a stage of development is probably 



