1 1 12 JOHN LEWIS BREMER 



In this paper and by means of the following reconstructions 

 made from serial sections of embryos in the Harvard Embryo- 

 logical Collection, I hope to show clearly that the view of His and 

 his supporters is in the main correct, that the cranial part of the 

 aorta arises as an extension of the capillary network of the yolk- 

 sac ; and also to throw more light on the development of the ven- 

 tral aorta, the aortic arches, and the pulmonary artery. For the 

 study of this question I have chosen to work primarily with the 

 rabbit, partly because of the excellence of this material in this 

 laboratory, and partly because the presence of the 'lateral hearts,' 

 described by Rathke, and easily recognizable in this species even 

 in early stages, readily marks the position of this part of the blood- 

 vessel net, and makes interpretation of the secondary foldings 

 much simpler. 



To some extent this same material was used by Dr. F. T. Lewis 

 in a paper on "The Intra-embryonic Blood-vessels of Rabbits 

 from 8§ to 13 days," which, accompanied by a demonstration of 

 sections and graphic reconstructions, was read at the meeting 

 of the American Association of Anatomists in 1903, but never 

 published in full. In the report of the Proceedings in The Amer- 

 ican Journal of Anatomy, vol. 3, a resume is given as follows: 

 ' ' From the network of vessels in the splanchnopleure of the yolk- 

 sac, all intra-embryonic vessels are apparently derived as off- 

 shoots. The network ends mesially in the two aortae. With 

 the formation of the pharynx, this net is so folded as to produce 

 dorsal and ventral aortae with the connecting first arch." It 

 will be seen that Lewis agrees with His as to the origin of the 

 dorsal aorta, but discards the idea that this vessel grows forward 

 around the tip of the pharynx to join ventrally with the anterior 

 end of the heart. A glance at figs. 1 and 3 will show that this 

 is correct ; dorsal aorta, first arch, ventral aorta and heart anlage 

 are all laid down almost simultaneously. 



Here a few words are needed on the character of the early blood- 

 vessels. The most recent investigations in this field have been 

 carried on almost exclusively by careful injections of fresh embryos, 

 which are then studied as transparent objects or are converted 

 into sections from which reconstructions are made. This method 



