154 EMBRYOLOGY. 



Besides the cell-layers which arose by invagination there had been 

 assigned to the middle germ-layer cells which detach themselves 

 individually from the primary germ-layers, and give rise between 

 the epithelial layers of the body to the snstentative substances, and 

 also to the blood, when such exists. Embryonic cells of that kind, 

 which are formed by emigration into the space surrounded by 

 the germ-layers, we named the mesenchymatic germ, and the tissue 

 produced from them mesenchyme. This occurs as well in two- 

 layered as in four-layered animals. In our opinion a sharp distinction 

 must be made between the formation of germ-layers, which is 

 correlated with the morphological differentiation of the body, and 

 the formation of mesenchyme, which will especially engage our 

 attention in one of the next chapters, if clearness and a uniform 

 principle are to be introduced into the whole germ-layer theory. 



In the second article it was our aim to show that in the Vertebrates 

 a middle germ-layer is developed by infolding. For that purpose 

 the development of Amphibia, Fishes, Reptiles, Birds, and Mammals 

 was compared with the development of Amphioxus, and thus was 

 acquired the foundation upon which is based the account of the 

 development of the middle germ-layer given in the preceding chapter. 



After the publication of these two papers, there appeared numerous 

 articles by VAN BENEDEN, DUVAL, HEAPE, HOFFMANN, KOLLIKER, 

 KOLLMANN, RABL, RUCKERT, STRAHL, WALDEYER, and others, through 

 which valuable facts concerning the development of the middle germ- 

 layer in the different classes of Vertebrates have been made known. 

 In some of these the chief points of view of the coelom-theory were in 

 general recognised as correct, attempts were made to modify details, 

 but especially was the question of the formation of the mesenchyme 

 of the Vertebrates actively discussed. 



The mechanical principle of the process of development, by means of 

 which the germ-layers are formed, and out of these the separate organs, 

 is appreciated in its full significance by only a few, and in text-books 

 particularly has not been adequately presented. 



Among the founders of the germ-layer theory, PANDER best com- 

 prehended this principle. " The blastoderm," he says in one place, 

 " forms, exclusively through the simple process of folding, the body 

 and the viscera of the animal. A delicate thread attaches itself as 

 the spinal cord to it, and scarcely has this taken place, when the 

 blastoderm sends the first folds, which themselves necessarily designate 

 the position of the spinal cord, as an envelope over the exquisite fila- 



