His doctrine, though it remains essentially unchanged, receives in his later works a less formal and more concrete expression, and, in particular, his views on the biogenetic law undergo some small modification. Even in the _General Morphology_ Haeckel had recognised that ontogeny is neither a complete nor an entirely accurate recapitulation of phylogeny; he had admitted, following F. Mueller, that the true course of recapitulation was frequently modified by larval and foetal adaptations. As time went on, he was forced to hedge more and more on this point, and finally in his _Anthropogenie_ (1874) and his second paper on the Gastraea theory (1875),[378] he had to work out a distinction between palingenetic and cenogenetic characters, of which much use was made by subsequent writers. The distinction may be given in Haeckel's own words:--"Those ontogenetic processes," he writes, "which are to be referred immediately, in accordance with the biogenetic law, to an earlier completely developed _independent ancestral form_, and are transmitted from this by _heredity_, obviously possess _primary_ importance for the understanding of the casual-physiological relations; on the other hand, those developmental processes which appear subsequently through _adaptation_ to the needs of embryonic or larval life, and accordingly can _not_ be regarded as repeating the organisation of an earlier independent ancestral form, can clearly have for the understanding of the ancestral history only a quite subordinate and _secondary_ importance. "The first I have named _palingenetic_, the second _cenogenetic_. Considered from this critical standpoint, the whole of ontogeny falls into two main parts:--First, _palingenesis_, or 'epitomised history' (_Auszugsgeschichte_), and second, _cenogenesis_, or 'counterfeit history' (_Faelschungsgeschichte_). The first is the true ontogenetic epitome or short recapitulation of past evolutionary history; the second is the exact contrary, a new foreign ingredient, a falsifying or concealing of the epitome of phylogeny."[379] As examples of palingenetic processes in the development of Amniotes, for instance, may be quoted the separation of two primary germ-layers, the formation of a simple notochord between medullary tube and alimentary canal, the appearance of a simple cartilaginous cranium, of the gill-arches and their vessels, of the primitive kidneys, the primitive tubular heart, the paired aortae and the cardinal veins, the hermaphroditic rudiment of the gonads, and so on. Cenogenetic processes, on the other hand, include such phenomena as the formation of yolk and the embryonic membranes, the temporary allantoic circulation, the navel, the curved and contracted shape of the embryo, and the like. The most important phenomena to be included under 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.