AORTA AND AORTIC ARCHES IN RABBITS 119 



reconstruction does not include the region of the segments. 

 Cephalad, this raised edge, the top of which is indicated by the 

 dotted line, makes a wide sweep laterally to form the future 

 optic vesicles, and extends over the region of the coelom; so that 

 a wide portion of the net may remain here, and may join with 

 the vessels under the coelom. Thus the first aortic arch is formed, 

 a net of vessels and cords connecting the dorsal aortic net with 

 the anterior end of the lateral heart. 





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Fig. 4 Rabbit, 8£ days, H.E.C. no. 624, section 101. Section through plane 

 .r - y in fig. 3. Md.Gr., medullary groove; Coe., coelom; Ao.d., three strands of 

 network for dorsal aorta; a, isolated portion of angioblast; b, part of network; Ve., 

 lateral heart. X 175. 



Of the original net of angioblast cords between the lateral heart 

 and the aortic net almost nothing remains. Here and there a 

 few isolated cells can be found, not connected with the rest of 

 the network; one of these is shown in fig. 4, others, though recog- 

 nized, were left out of the reconstruction for the sake of clearness. 

 Other cords have also been lost; of those connecting the lateral 

 heart with the extra-embryonic angioblast only the most caudal 

 remains, the others have either entirely disappeared or have lost 

 their lateral connections. The cause of this latter destruction 

 of the net is shown in fig. 4 ; the lateral heart, covered by a reflec- 

 tion of the mesoderm, has expanded so far into the coelom that 

 vessels from it to the lateral net must take a curving course, and 

 would probably be compressed between mesoderm and entoderm. 

 This has occurred progressively from before backward, until the 

 lateral heart gradually leaves the region of the coelom; here no 



