HISTOKT OF THE GERM-LAYEE THEORY. 145 



First, the body-sacs are detached from the fundament of the 

 chorda and the entoblast, whereupon the edges of the 

 parietal and visceral lamellae, thus set free, fuse with 

 each other. 



Secondly, the fundament of the chorda is bent into a chordal 

 groove, and this is converted into a solid rod, -which is 

 ^ completely isolated from the entoblast. 



Thirdly, the entoblast closes together into a tube with a dorsal 

 raphe. 



5. The development of the three fundaments, as also that of 

 various other organs, begins at the head-end of the embryo, and 

 advances from here toward the blastopore, where for a long time a 

 continual formation of new parts and an increase in the longitudinal 

 growth of the body take place. 



6. During the development of the middle germ-layer, the blasto- 

 pore of the Amphibians, Fishes, Reptiles, Birds, and Mammals has 

 been metamorphosed into a groove occupying the longitudinal axis 

 of the embryo (primitive groove of the higher Vertebrates). 



7. The blastopore and the primitive groove in later stages of 

 development undergo degeneration, and are not converted into any 

 organ of the adult. (For the details of this, see Part II.) 



8. Before their disappearance the blastopore and primitive groove 

 are surrounded by the medullary folds and taken into the terminal 

 part of the neural tube, whereby a direct communication between 

 neru'al tube and intestinal tube — the neurenteric canal — is effected. 

 The two organs, which communicate with each other for a long time, 

 are later separated by its closure. 



CHAPTER YII. 



HISTORY OF THE GESM-LATES. THEORY. 



The fundamental facts of the sheet-like structure of the vertebrate 

 body, which have been treated of in the two preceding chapters, are 

 epitomised as the doctrine of the germ-layers, or the germ-layer 

 theory. Since this theory is of the most far-reaching significance, 

 for the comprehension of the evolution of form in animals, and can 

 be placed side by side with the cell-theory as coequal with the latter, 

 I devote a separate chapter to its history. 



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