EMBRYOLOGY 



In Fig. 230 the parts are still further developed. The neural canal 

 is represented (A) nearly the same as in Fig. 229, with the notochord 

 (E) just beneath it. A groove, or gutter (D) has been formed in front, 

 which is the groove of the intestinal canal, which remains open at this 

 time and is lined with the hypoblast. Just above D is a single opening 

 (G), which is formed by the union of the two openings (G, G) in Figs. 

 228 and 229; and this is the abdominal aorta, which has here become 

 single. The two openings (H, H) represent a section of the body-cavity. 

 The outer wall of this cavity is the outer visceral plate, which is devel- 

 oped 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 mesoblast, and 

 the inner wall is the inner layer of the same membrane. The two 

 round openings (i, i) are sections of the Wolffian ducts (see Plate XV, 



Fig. 6). 



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 dorsal 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 verte- 

 bral plates ; and at about the same time, the 

 Fig. 230. Later stage of de- two aortae make their appearance, with the 

 veiopmcnteftk* chick ($**>v&v*& fi rst traces of the body-cavity (celom). The 



Briicke). J ^ 



enclosure of the neural canal takes place 



from before backward, beginning at the cephalic end. The canal re- 

 mains open at the caudal end for some time after it has been enclosed 

 above, as is shown in Figs. 228 and 229. 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 embryo is open in 

 front. In the further processes of development, the visceral, or 

 abdominal plates advance and close over the abdominal cavity, in 

 the same way as the medullary plates have closed over the neural 

 canal. . Thus there is formed a closed tube, the intestine, which is lined 

 with 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 ab- 

 domen, 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. 



