ix GERM LAYER THEORY 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 Remak. 

 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. Remak (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 Allman " 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 



