DEVELOPMENT OF THE EMBRYON. 



913 



figure, the superior layer of dark cells (5, &) represents the external blastodermic mem- 

 brane. The inferior layer of dark cells (d, d) represents the internal blastodermic mem- 

 brane. The middle layer of lighter cells is the intermediate membrane, which, toward 



FIG. 294. 



the periphery, is split into two layers. This figure represents a transverse section. At 

 , 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) represent the situation of the two aortae. The other cavities 

 are as yet indistinct in this figure. 



FIG. 295. 



Fig. 295 shows the same structures at a more advanced stage of development. The 

 dorsal, or vertebral plates, which bound the furrow (a) in Fig. 294, are closed above, 

 and include (a) the neural canal. The chorda dorsalis (e) is separated from the cells sur- 

 rounding it in Fig. 294. "We have still the external blastodermic membrane (5, J) and the 

 internal blastodermic membrane (d, d), presenting various curves which follow the arrange- 

 ment of the cells of the intermediate layer. By 

 the sides of the boundaries of the neural canal, 

 are two distinct masses of cells (c, c), which are 

 developed into the vertebrae. Outside of these 

 masses of cells, are two smaller collections of 

 cells, afterward developed into the Wolffian 

 bodies, which will be described farther on. 

 Beneath those two masses, are two large cavi- 

 ties (gr, g), the largest cavities shown in Fig. 

 295, presenting an irregular form, which are 

 sections of the two primitive aortas. The two 

 openings (A, 7i) become afterward the pleuro- 

 peritoneal cavity. 



In Fig. 296, the parts are still farther de- 

 veloped. The neural canal is represented (a) 

 nearly the same as in Fig. 295, with the chorda 

 dorsalis (e) just beneath it. A groove, or gutter 

 (d) has been formed in front, which is the groove of the intestinal canal. This remains 

 open at this time and is lined by the internal blastodermic membrane. Just above d, is 

 58 



ff 



FIG. 296. 



