390 



GENERATION. 



merit in the chick. In this figure, the superior layer of dark 

 cells (J) represents the external blastodermic membrane. The 

 inferior layer of dark cells (d) represents the internal blasto- 

 dermic membrane. The middle layer of lighter cells is the 

 intermediate membrane, which, toward the periphery, is split 



FIG. 37. 



into two layers. This figure represents a transverse section. 

 At #, is a transverse section of the groove which is subse- 

 quently developed into the canal for the spinal cord. Be- 

 neath this groove, is a section of a rounded cord (#), the 

 chorda dorsalis. The openings (g) represent the situation of 

 the two aortse. The other cavities are as yet indistinct in 

 this figure, 



FIG. 38. 



Fig. 38 shows the same structures at a more advanced 

 stage of development. The dorsal, or vertebral plates which 

 bound the furrow (a) in Fig. 37, are closed above, and in- 

 clude (a) the neural canal. The chorda dorsalis (e) is sepa- 

 rated from the cells surrounding it in Fig. 37. We have still 



