58 AORTIC-AKCH SYSTEM IN THE HUMAN EMBRYO. 



extremities as parts of the so-called adult sixth. This suggests a possible signifi- 

 cance in the fact that in man the so-called fifth arch enters the pulmonary arch 

 close to its upper end. Of the 6 well-developed so-called fifth arches that have been 

 described in the human embryo, 5 enter the pulmonary near its termination. If it 

 be established that these vessels are true fifth arches, their usual termination would 

 indicate strongly that the upper end of the pulmonary arch is the homologue of 

 the distal portion of the fifth. 



Not only is the status of the channel lying between the fourth and last aortic 

 arches unsettled, but the pulmonary arch also depends on a more complete under- 

 standing of the caudal pharyngeal complex for its interpretation. Shaner has 

 recently shown that in the turtle the terms sixth arch and pulmonary arch are 

 not necessarily synonymous. He finds an arch caudal to the fifth, which gives 

 off the primitive pulmonary artery but still is not the equivalent of the human 

 pulmonary arch, since it lies craniolateral instead of caudomedial to the caudal 

 pharyngeal complex. At the same time the equivalent of the human pulmonary 

 arch is indicated by a spur from the upper end of this vessel curving around to the 

 caudomedial side of the complex. 



VENTRAL CONNECTIONS OF AORTIC ARCHES AORTIC SAC. 



The literature concerning the nature of the ventral connections of the heart 

 and branchial arterial arches shows a surprisingly great diversity of view, con- 

 sidering the numerous accounts of vascular development. The terminology of 

 this region is in a correspondingly unsatisfactory state. Few authors are in com- 

 plete agreement in the use of such fundamental terms as aortic trunk, bulb, or 

 ventral aorta, and we still find in recent editions of our anatomical texts portions of 

 the paired dorsal aortse referred to as parts of aortic arches, as in the time of Rathke 

 and von Baer. 



In the mammalian embryo a saccular enlargement intermediates between the 

 aortic arches and trunk. A slight swelling can be made out at the junction of the 

 first arches and trunk in the human embryo even before the second arch is estab- 

 lished (fig. 1). It reaches its highest development when giving origin to the third, 

 fourth, and pulmonary arches and before it has begun to separate into its aortic 

 and pulmonary divisions (figs. 5, 6). At this time it is decidedly flattened dorso- 

 ventrally and the arches radiate from it. It varies greatly in form, corresponding 

 to the tendency of this region to be drawn out in either its craniocaudal or trans- 

 verse axis, and also in response to fluctuations in the form of the individual pouches 

 and arches. The cleft between the points of origin of the fourth and pulmonary 

 arches begins to deepen soon after the caudalmost arch is completed. Before the 

 branchial stage is at an end the sac has separated completely into aortic and 

 pulmonary portions. The pulmonary division is tubular but the part that gives 

 rise to the third and fourth arches is for a time still somewhat flattened and sac- 

 like. 



The enlargement at the origin of the arches is not confined to mammalian 

 embryos. Greil (1903), in his work on the development of the truncus arteriosus 

 in Anamnia, finds a similar chamber in Acanthias embryos and Salamandra 



