ix GEEM LAYEE THEOEY 505 



_ Addendum to Chapter IX.— More than once in the course of 

 this volume reference has been made to the " Theory of Germinal 

 Layers " or the " Germ Layer Theory." This theory, which has 

 played a great part in the development of embryological science in 

 the past and still dominates to a great extent embryological research, 

 had its foundations in observations made by these pioneers of 

 embryological science — Wolff, Pander, von Baer and Eemak. 

 Wolff (1768) observed that the alimentary canal in the Bird embryo 

 is developed out of a thin membrane or leaf (" Blatt ") and inferred 

 that the other organs go through a similar stage. Pander (1817) 

 gave the name " blastoderm " to the first membrane-like stage of the 

 embryo as a whole, saw how this became differentiated into the three 

 layers — outer, middle and inner — and traced out the development 

 from these of the main organ-systems. Von Baer (1828) carried on 

 and elaborated Pander's work, recognized that the middle layer was 

 double, and that it was secondary to the two primary layers : the 

 outer and the inner. He also extended his observations to forms 

 other than the Fowl and laid the foundations of Comparative Embry- 

 ology. Eemak (1855) finally worked out the germ-layers in terms 

 of the Cell-theory, traced the origin of the coelome to a split in the 

 middle layer, and worked out more precisely the relations of the 

 layers to the definitive organ-systems. 



One of the most important steps in the development of the Germ 

 Layer Theory was made by Huxley (1859) who as a result of his 

 researches upon the Medusae recognized the two primary cell-layers 

 in these animals (named by Airman " ectoderm " and " endoderm ") 

 and suggested the comparison of them with the two primary layers 

 of the Vertebrate embryo. 



Embryology, like Morphology in general, first became a real 

 living science as a result of Darwin's demonstration of the fact of 

 evolution. In the Origin of Species (1859) the principle of recapitu- 

 lation is already admitted. " Embryology rises greatly in interest, 

 when we thus look at the embryo as a picture, more or less obscured, 

 of the common parent-form of each great class of animals." The 

 idea was further elaborated by Fritz Miiller (1864). 



Kowalevsky (1871, etc.) and other embryologists had demon- 

 strated the wide-spread occurrence among the Invertebrates of an 

 early stage of development more or less cup-shaped in form and 

 consisting only of the two primary cell-layers, and the important 

 advance was made synchronously by Lankester and Haeckel of 

 perceiving in this two-layered stage a repetition of a common 

 ancestral form. 



Lankester (1873) recognized amongst the Metazoa two distinct 

 grades of complexity of structure so far as their cell-layers were 

 concerned — the diploblastic grade (represented by the Coelenterate) 

 consisting of the two primary layers, and the triploblastic grade with 

 an interposed middle layer. Further he recognized that each 

 Metazoon — whatever its definitive condition — passes in the course 



