DEVELOPMENT OF THE OVUM. 815 



Fig. 299, presenting an irregular form, which are sections of the two primi- 

 tive aortae. The two openings (H, H) afterward become the pleuro-peritoneal 

 cavity. 



In Fig. 300 the parts are still farther developed. The neural canal is 

 represented (A) nearly the same as in Fig. 299, 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 hypoblast. Just above D, is a 

 single opening (G), which is formed by 

 the union of the two openings (Gr, G) in 

 Figs. 298 and 299 ; and this is the ab- 

 dominal aorta, which has here become 

 single. The two openings (H, H) rep- 

 resent a section of the pleuro-peritoneal 

 cavity. The outer wall of this cavity is 

 the outer visceral plate, which is devel- 

 oped into the muscular walls of the ab- 

 domen. 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 mesoblast, and the inner wall is the inner layer of the same membrane. 

 The two round orifices (I, I) are sections of the Wolffian ducts. The space 

 (#, Z>) is the amniotic cavity. 



The figures just described, it must be borne in mind, represent transverse 

 sections of the body of the chick, made through the middle portion of the 

 abdomen. The posterior parts, it is seen, are developed first, the situation of 

 the vertebral column being marked soon after the enclosure of the neural 

 canal, by the vertebral plates ; and at about the same time, the two aortae 

 make their appearance, with the first traces of the pleuro-peritoneal cavity. 

 The next organs in the order of development, after the vascular system, are 

 the Wolffian bodies. The intestinal canal is then a simple groove, and the 

 embryon is entirely open in front. In the farther process of development, 

 the visceral plates advance and close over the abdominal cavity, as the 

 medullary plates have closed over the neural canal. Thus there is formed a 

 closed tube, the intestine, lined by the hypoblast, the walls of the intestine 

 being formed of the inner layer of the mesoblast. This brings the external 

 layer of the mesoblast around the intestine, to form the muscular walls of the 

 abdomen, the cavity (Fig. 300, H, H) being the peritoneal cavity, and the 

 external covering being the epiblast. At this time the- Wolffian bodies lie 

 next the spinal column, between the intestine and the abdominal walls, with 

 the single, abdominal aorta situated behind the intestine. 



DEVELOPMENT OF THE SKELETON, MUSCULAR SYSTEM AND SKIN. 



Chorda Dorsalis. One of the earliest structures observed in the develop- 

 ing embryon is the chorda dorsalis, or notochord. This is situated beneath 



