BLASTULA, GASTRULA, AND GERM LAYERS 357 



Metazoan phyla. Exceptions and contradictions were indeed 

 occasionally noted, but their importance was minimized and 

 they were treated frankly as exceptions, and put down to the 

 account of " coenogenetic " modifications of " palingenetic " 

 characteristics (see Chapter I). 



It is difficult to overestimate the influence of this theory upon 

 the history of Embryology, and upon fundamental embryo- 

 logical ideas. Perhaps no conception, other than the general 

 theory of evolution, has had greater influence in the field of 

 descriptive embryology. 



More recently, however, the limitations in the general 

 applicability of this theory have been more fully recognized 

 and the exceptions to the validity of its essential ideas empha- 

 sized. At present we must recognize the germ layers as 

 representing a stage in development, just as do the blastula or 

 gastrula, and of no greater or lesser importance than these. 

 The germ layers are descriptive terms of the greatest .impor- 

 tance, as such they are indispensable. They are not, however, 

 starting points in any real sense; and to regard them as such is 

 to look forward merely, not backward. Looking both forward 

 and backward we see that the establishment of the germ layers 

 is only one step in the continuous process of development. 

 They represent no more 'essential homologies than many other 

 features held in common by many developing organisms. 



While we cannot consider in extenso the facts which have led 

 to this change of opinion regarding the importance of the germ 

 layers, we are bound to state the nature of certain classes of 

 these facts. In the first place are to be noted the difficulties 

 of homologizing the layers of certain groups with their typical 

 condition. For example, in the Porifera that layer which seems 

 entitled to be termed the ectoderm, really gives rise to struc- 

 tures ordinarily derived from endoderm, while the "endoderm" 

 itself forms the covering tissues. In the Mammals the "ecto- 

 derm" may contribute little or nothing to the formation of the 

 real embryo and the inner, outer, and middle layers cannot 

 be exactly homologized with those of other Chordates, save by 

 the grace of terminology. In the earlier part of this chapter 



