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GENERATION. 



an introduction to a description of the development of special organs in the 

 human subject and in mammals, it will be found very useful to study the 

 first stages of development in the chick, which will give an idea of the ar- 

 rangement of the different blastodermic layers and the way in which they 

 are developed into the different parts of the trunk, with the mode of forma- 

 tion of the great cavities. The figures by which this description is illustrated 

 are those of Briicke, which were photographed on wood from diagrams made 

 from actual preparations by Seboth. These figures, therefore, can hardly be 

 called diagrammatic. 



Fig. 298 shows one of the earliest stages of development in the chick. In 

 this figure, the upper layer of dark cells (B, B) represents the epiblast. The 



lower layer of dark cells (D, D) represents the hypoblast. The middle layer 

 of lighter cells is the mesoblast, which, toward the periphery, is split into two 

 layers. This figure represents a transverse section. At A, is a transverse 

 section of the groove which is subsequently developed into the canal for the 

 spinal cord. Beneath this groove, is a section of a rounded cord (E), the 

 chorda dorsalis. The openings (G, G) represent the situation of the two 

 aortse. The other cavities are as yet indistinct in this figure. 



Fig. 299 shows the same structures at a more advanced stage of develop- 

 ment. The dorsal, or vertebral plates, which bound the furrow (A) in Fig. 



FIG. 299. 



298, are closed above, and include (A) the neural canal. The chorda dorsalis 

 (E) is separated from the cells surrounding it in Fig. 298. The epiblast 

 (B, B) and the hypoblast (D, D) present certain curves which follow the 

 arrangement of the cells of the mesoblast. By the sides of the boundaries 

 of the neural canal, are two distinct masses of cells (C, C), which are devel- 

 oped into the vertebrae. Outside of these masses of cells, are two smaller 

 collections of cells, afterward developed into the Wolffian bodies. Beneath 

 those two masses, are two large cavities (G, G), the largest cavities shown in 



