Embryology. 131 



also gives rise to the embryonic axis or notochord; 

 while the rest of the body (such as muscles and skele- 

 ton) is mainly due to a third stratum of cells (meso- 

 derm), which usually arises between the ectoderm and 

 the endoderm. 



For many years embryologists, from Von Baer on- 

 wards, were much concerned with the origin of these 

 germinal layers, and with showing how they gave rise, 

 separately or in combination, to the various organs of 

 the body. It was held to be one of the criteria of com- 

 plete homology, that anatomically similar organs should 

 be traceable to an origin in similar layers. It was held 

 that homology must be corroborated by " homodermy", 

 and the fundamental similarity of the germ -layers 

 throughout the Metazoa was the keystone of the so- 

 called germ-layer theory (Keimblattertheorie)\ and it 

 was in this connection a step of historical importance 

 when Huxley (1849) collated the epiblast and hypoblast 

 of the embryo with the two layers of cells which are 

 seen in the structure of an adult polype, like the com- 

 mon hydra. 



Gradually, however, the confidence of embryologists 

 in this germ-layer theory has been shaken by the fol- 

 lowing, among other considerations, (a) What one 

 may call the stratification of the embryo is established 

 in very different ways in different types; (b) there are 

 some cases, notably sponges, where the products of the 

 ectoderm and the endoderm cannot be readily brought 

 into line with the state of affairs in the majority ; (c) the 

 mesoderm is so varied in its origin (from ectoderm, 

 endoderm, or both), and in its occurrence, that the 

 conception lacks even a pretence at unity; (d) in many 

 cases the facts of development show that certain organs 

 can be traced back to a few cells, specifically predes- 

 tined from their first appearance, rather than to a homo- 

 geneous layer. 



" It has become", E. B. Wilson says, "more and 

 more clear that the germ-layer theory is, to a certain 

 extent, inadequate and misleading, and that even the 

 primary layers of the ' gastrula ' cannot be regarded as 

 strictly homologous throughout the animal kingdom, 



