HAECKEL: CENOGENESIS 259 



the general heading of cenogenesis are, first, the occurrence 

 of food-yolk, and second, those anomalies of development 

 which are classed by Haeckel as heterochronies and 

 heterotopies. 



It is to the influence of the different amounts of yolk 

 present in the egg that are due the great differences in 

 the segmentation and gastrulation processes, which almost 

 mask their true significance. 



Heterochronic processes are such as arise through the 

 dislocation of the proper phylogenetic order of succession : 

 heterotopic processes in the same way are caused by a 

 wandering of cells from one germ-layer to another. The two 

 classes of phenomena are disturbances either of the proper 

 spatial or of the proper temporal relation of the parts during 

 development. 



Heterochrony shows itself, as a rule, either as an accelera- 

 tion or as a retardation of developmental events,' as compared 

 with their relative time of occurrence during phylogeny. 

 Thus the notochord, the brain, the eyes, the heart, appear 

 earlier in the ontogenetic than in the phylogenetic series, 

 while, on the other hand, the septum of the auricles appears 

 in the development of the higher Vertebrates before the 

 ventricular septum, which is undoubtedly a reversal of the 

 phylogenetic order. 



Cases of heterotopy, or of organs being developed in a 

 position or a germ-layer other than that in which they 

 originally arose in phylogeny, are not so easy to find. 

 According to Haeckel, the origin of the generative products 

 in the mesoderm is a heterotopic phenomenon, for he 

 considers that they must have originated phylogenetically in 

 one of the two primary layers, ectoderm or endoderm. 



It is worthy of note that the help of comparative anatomy 

 is admittedly required in deciding what processes are 

 palingenetic and what cenogenetic (p. 412). 



Haeckel's morphological notions, and particularly his 

 biogenetic law, excited a good deal of adverse criticism 

 from men like His, Claus, Salensky, Semper and Goette. 

 Nor was his principal work, the General Morphology, received 

 with much favour. Nevertheless, since he did express, 



