FIRST DEVELOPMENT OF THE EMBBYON. 



391 



FIG. 89. 





the external blastodermic membrane (b) and the internal blas- 

 todermic membrane (d), presenting various curves which fol- 

 low the arrangement of the cells of the intermediate layer. 

 By the sides of the boundaries of the neural canal, are two 

 distinct masses of cells (<?), which are developed into the ver- 

 tebrae. Outside of these masses of cells, are two smaller col- 

 lections of cells, afterward developed into the Wolffian bodies, 

 which will be described farther on. Beneath -these two 

 masses, are two large cavities (g\ the largest cavities shown 

 in Fig. 38, presenting 

 an irregular form, which 

 are sections of the two 

 primitive aortse. The 

 two openings (A) become 

 afterward the pleuro-pe- 

 ritoneal cavity. 



In Fig. 39, the parts 

 are still farther devel- * 

 oped. The neural canal 

 is represented (a) nearly 

 the same as in Fig. 38, 

 with the chorda dorsalis 

 (e) just beneath it. A. 

 groove, or gutter (d) has 



been formed in front, which is the groove of the intes- 

 tinal canal. This remains open at this time, and is lined 

 by the internal blastodermic membrane. Just above d, is 

 a single opening (g\ which is formed by the union of the 

 two openings (g) in Figs. 37 and 38, and this is the abdomi- 

 nal aorta, which has here become single. The two openings 

 (A) represent a section of the pleuro-peritoneal cavity. The 

 outer wall of this cavity is the outer visceral plate, which is 

 developed into the muscular walls of the abdomen. The 

 lower and inner wall is the inner visceral plate, which forms 

 the main portion of the intestinal wall. The outer wall is 

 the outer layer of the intermediate membrane, and the inner 



