We must, above all, clearly distinguish between palingenetic and cenogenetic phenomena.
Thorough opponents of the biogenetic law have laid great stress on these and similarcenogenetic phenomena, and endeavoured to urge them as striking disproofs of the law.
In these three classes alone we find the remarkable embryonic membrane, already mentioned, which we called the amnion; a cenogenetic adaptation that we may regard as a result of the sinking of the growing embryo into the yelk-sac.
In order to distinguish correctly between palingenetic and cenogenetic phenomena in embryology, and deduce sound conclusions in connection with stem-history, we must especially make a comparative study of the former.
There is an interesting cenogenetic phenomenon in the formation of the heart of the higher Vertebrates that deserves special notice.
Moreover, in the animals in which we do not find a real palingenetic blastula the defect is clearly due to cenogenetic causes, such as the formation of food-yelk and other embryonic adaptations.
In this we have throughout taken strict account of the distinction between palingenetic and cenogenetic phenomena.
The cenogenetic modifications of the latter are more appreciable the more food-yelk is stored up in the ovum.
These cenogenetic phenomena are foreign or later additions; they allow us to draw no direct inference whatever as to corresponding processes in our ancestral history, but rather hinder us from doing so.
With the formation of the primitive gut our frog-embryo has reached the gastrula stage, though it is clear that this cenogenetic amphibian gastrula is very different from the real palingenetic gastrula we have considered (Figures 1.
The cenogenetic chordula-forms of the craniotes must therefore be derived from the palingenetic embryology of the amphioxus in the same way as I had previously proved for their gastrula-forms.
By far the most important process that determines the various cenogenetic forms of gastrulation is the change in the nutrition of the ovum and the accumulation in it of nutritive yelk.
It is the more instructive as even cenogenetic structures may in certain circumstances have a high phylogenetic value.
These cenogeneticmodifications seem to be so great that until twenty years ago these important processes were totally misunderstood.
Of these four forms we must regard one only as the original palingenetic, and the other three as cenogenetic and derivative.
It is worthy of note that the help of comparative anatomy is admittedly required in deciding what processes are palingenetic and what cenogenetic (p.
This typical formation might be masked by cenogeneticmodifications caused chiefly by the presence of yolk.
These cenogenetic phenomena are later additions; we must not infer from them that there were corresponding processes in the ancestral history, and hence they are apt to mislead.
Besides, the special care of the young on the part of the amniotes gives their embryo much more favorable conditions for the cenogeneticabbreviation of the palingenetic evolution.
Their evolution is subject to great cenogenetic modification in man and all other viviparous animals, precisely because the complete development of the embryo occupies a longer time within the body of the mother.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "cenogenetic" in a variety of sentences. We hope that you will now be able to make sentences using this word.
