124 EDWARD PHELPS ALUS jr., 



develops into the heart. Proceeding backward and upward this median 

 vessel almost immediately separates into two parts, one on each side. 

 A large aortic arch is then sent upward , from each of these two 

 vessels, in front of a solid mass of cellular tissue, continuous with 

 the entoderm, which reaches to and is in contact with the ectoderm, 

 and incloses, in its posterior part, the slight evagination of the spira- 

 cular cleft. This tissue I suppose to be entirely of entodermal origin 

 and I shall hereafter refer to it as the spiracular outgrowth, or cord. 

 Dorsally this aortic arch joins the carotid artery, having first, about 

 midway in its course, sent a large branch backward, lateral to and 

 beneath the tissues of the spiracular outgrowth. Posterior to the 

 spiracular outgrowth this branch vessel turns mesially and joins the 

 carotid. As it here turns mesially there was , on one side of the 

 specimen , a small bud projecting downward. After giving off this 

 aortic arch the ventral vessel continues backward, and about opposite 

 the hind edge of the spiracular outgrowth sends a relatively small 

 branch laterally , and then upward for a short distance , where it 

 disappears. Further backward the first branchial aortic arch is given 

 off". It is thus evident that there is, in this specimen, a large and 

 complete prespiracular aortic arch; that there is a postspiracular 

 aortic arch developed in its dorsal and ventral portions but not per- 

 ceptibly developed in its middle portion ; and that these two arches 

 are connected by a commissure lying morphologically ventral to the 

 spiracular cleft. Either that, or the pre- and post-spiracular arches 

 have fused in their ventral portions and are only separate and distinct 

 in their dorsal portions. This latter supposition seems wholly im- 

 probable, but my material and preparations are not at present suitable 

 for its further! investigation. 



In a 7 mm embryo (Fig. 2) the conditions were the same as in 

 the 6 mm one excepting that the postspiracular aortic arch was a 

 complete arch, the dorsal bud and small ventral vessel found in the 

 6 mm embryo being replaced by a continuous vessel. The connection 

 of the median ventral vessel with the heart had, in this specimen, 

 shifted backward slightly beyond the anterior end of the ventral 

 vessel. Immediately posterior to the heart, and at the point where 

 the first aortic arch is given off", the median ventral vessel separated 

 into two parts, one on each side. 



In 8 mm embryos, of which I have several series of somewhat 

 imperfect sections, the conditions are the same as in 7 mm ones, ex- 



