100 AORTIC-ARCH SYSTEM IN THE HUMAN EMBRYO. 



the pulmonary arch has become a continuous channel. Later, the third ceases 

 to send blood caudalward into the aorta. Thus the stream from heart to aorta is 

 shifted caudally in the branchial period. After the pulmonary arch is complete 

 there is a period of comparative stability of the arches, the fourth and pulmonary 

 delivering the blood caudally into the aorta, aided for a time by the third. The 

 length of the third and fourth arches is almost constant throughout their existence, 

 because their form is dependent on the caudal portion of the pharynx, which in- 

 creases in size very little after its establishment and early shows regressive changes. 



Fifth Arch. 



Loops, or so-called "island-formations," appear sometimes in the angioblastic 

 and capillary net which precedes the upper end of all arches but the first. Occa- 

 sionally they are found at the lower end. Those found at the upper end of the 

 fourth and pulmonary arches have been incorrectly classed with other arterial 

 channels which bear a real resemblance to a fifth arch. If one omits the island- 

 formations, 6 cases of so-called fifth arches have been described in man. In 5 

 of these it arose from the lower end of the fourth arch or the subjacent sac and passed 

 to the upper end of the pulmonary arch. Lewis and Kingsbury point out that 

 even the occasional existence of a fifth arch can not be regarded as established, 

 since the identity of this structure depends on its lying in a fifth visceral arch, and 

 this has never been proved. Yet there is some evidence of the occasional occurrence 

 of the latter in a more or less complete form. Projecting from the aorta and sac 

 are frequently found sprouts corresponding to the upper and lower ends of the 

 fifth arches. Taken with the complete vessels, these were found in 50 per cent of 

 the embryos representing the time of establishment of the pulmonary arch and a 

 little later. It can not be said how many of these are developing so-called fifth 

 arches, how many are stages of regression, and how many are incompletely devel- 

 oped so-called arches which never will progress farther. 



The enlargement where the arches come off from the arterial trunk, which we 

 have termed the aortic sac, is already present when the second arch is forming. 

 It is best developed when the three pairs of arches are coming off from it and re- 

 mains for a time after the pulmonary arches and trunk are cut off. A similar 

 structure is found among anamniote embryos, and a sac of similar form and posi- 

 tion is observed in some adults of the same group. Perhaps the embryonic like the 

 adult sac either serves to distribute the diastolic pressure or is a mechanical adapta- 

 tion to the forces resulting from the rapid deflection of the current from the arterial 

 trunk into the arches. Dr. Streeter suggests, as a purely developmental explana- 

 tion for its presence, that it may be due to an excessive proliferation of endothelium 

 which is to be used up later in differentiation of the arteries. 



Most writers describe paired ventral aortse in the human embryo. There are 

 at different times a few temporary channels leading from the sac which, by their 

 approximately cranio-caudal course, resemble fragments of ventral aortae. Such 

 are the longitudinal segments that appear in the late history of the first and second 

 arches and the paired sprouts which give rise to the proximal parts of the pulmonary 



