The Devel()i)iiu'iit of the Heart in Shad. 235 



The aortic cells do not appear suddenly. At 26 somites (Figs. 

 6B, 6D, etc.) a break is apparent, in most sections, in the line of 

 the future dorsal aorta, (the breaks together forming a tunnel) the 

 cells about the break have small nuclei and a large amount of cyto- 

 plasm; at 32 somites the aortic cells are not only clearly endothelial 

 but tend to line the tunnel. In the light of later stages the break 

 referred to can be distinguished in many sections at 22 somites 

 (Fig. 5), but it is questionable whether the cells about it have under- 

 gone much differentiation. The differentiation of the cells of the 

 dorsal aorta thus goes hand-in-hand with that of the cells known to 

 be derived from the portion moyenne, and the dorsal aorta is recog- 

 nizable before the ventral aortic cells have taken up their definitive 

 position. Finally, the history of the carotid artery is similar to that 

 of the dorsal aorta, and, at 32 somites, the cells of the ventral and 

 dorsal aorta are continuous around the front of the mandibular pouch 

 through the mandibular aortic arch. 



This entire chain of circumstances suggests very strongly that 

 the dorsal aorta and carotid of the anterior region, like the ventral 

 aorta and endocardium, arise from the portion moyenne. 



In the middle region (of the three before ^mentioned) there is also 

 isolation of the cells adjacent to the medial borders of the lateral 

 plates to form the portion moyenne. Soon after the portion moyenne 

 (which is here less bulky than in the anterior region) ceases to be 

 recognizable the differentiation of the cells about to form the dorsal 

 aorta can be followed in exactly the same manner as in the anterior 

 region. Here are two sets of facts, the disappearance of the portion 

 moyenne, and the gradual differentiation of the dorsal aorta nearer 

 to the mid-line; they are, of course, not necessarily dependent on 

 one another, but considered in connection with the circumstances 

 in the anterior region they may be looked upon as suggestive. 



By reference to Fig. it will be seen that the dorsal aorta in 

 the anterior and middle regions is always situate medial to the line 

 of the lateral margin of the entodermal pharynx (usually very much 

 so). Figs. 3B, 3C, 3E and 3F show that the portion moyenne 

 is always in a position lateral to the above mentioned line. From 

 these two facts it follows that although the conditions in the anterior 



