This typical formation might be masked by cenogenetic modifications caused chiefly by the presence of yolk.    From Wordnik.com.  [Form and Function A Contribution to the History of Animal Morphology] Reference

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).    From Wordnik.com.  [Form and Function A Contribution to the History of Animal Morphology] Reference

In all other extent Vertebrates these fundamental processes have been more or less modified by adaptation to the conditions of embryonic development especially by changes in the food yolk; they exhibit various cenogenetic forms of the formations of the germlayers, and thus develop by means of a metagastrula.    From Wordnik.com.  [Haeckel on gastrulation - The Panda's Thumb] Reference

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. ".    From Wordnik.com.  [Evolution in Modern Thought] Reference

"In the evolutionary appreciation of the facts of embryology we must take particular care to distinguish sharply and clearly between the primary, palingenetic evolutionary processes and the secondary, cenogenetic processes.    From Wordnik.com.  [Evolution in Modern Thought] Reference