100 



EDWAED MCCRADY, JR. 



abundantly clear from other anatomical features. I am fairly 

 confident that they would never have been born, but would 

 have been completely resorbed during the next day or so of 

 gestation. 



Changes in the vascular system. The atria of the heart 

 tubes meet and fuse during stage 27 and the heart begins to 

 beat. From this time on corpuscles may be found in all the 

 blood vessels of the body. For the sake of clarity, however, 

 they have been omitted from the section tracings. 



26 



27 



Fig. 34 Dorsal views of embryos showing closure of the amniopore. 



In connection with the beginning of circulation of the blood 

 it is interesting to note how far the vascular system has al- 

 ready developed. In early specimens of stage 24, as already 

 described, there is on each side of the body a capillary plexus 

 leading from the anterior end of the heart tube through the 

 mandibular mesenchyme, then caudad to the end of the body 

 and out into the area vasculosa. In later specimens of the 

 same stage there has been a selection and enlargement of two 

 main arterial vessels from this plexus, the dorsal aortae, and 

 an elimination of most of the other original channels. By a 

 similar process in stage 25 the mandibular capillary plexus is 

 converted into two well-formed aortic arches. In stage 26 the 

 capillary plexus in the hyoid mesenchyme shows a reduction in 



