96 DEVELOPMENT OF PRIMITIVE BLOOD-VESSELS. 



The series of sections of injected chicks shown on plates 2, 3, and 4 allow an 

 interesting comparison of the primitive branches of the aorta. It is clear that the 

 branches of the aorta can be described best according to what organs they supply 

 rather than by regarding their exact point of origin in the wall of the aorta. The 

 primary branches extend to the splanchnopleure and are primitively directly 

 lateral branches, as can be seen on the left side of text-figure 3. That they come 

 to be ventro-lateral and then ventral branches is well known and is shown in 

 plate 2, figure 4, in which it is clear that there are three sets of arteries on the 

 right side of the section. The first is a dorsal branch to the cardinal vein; the 

 second is a lateral branch to the somatopleure, and the third is a ventral branch 

 to the splanchnopleure. The branches that extend to the cardinal veins need 

 careful attention. There are in the first place the original dorsal arches that 

 give rise to the cardinal veins, such as are shown in figure 4 of plate 2 and figure 4 

 of plate 3. These branches are strictly intersegmental ; moreover, the interseg- 

 mental branches are the first arteries related to the cardinal veins because they are 

 the only ones present at the stage of 12 somites. In later stages (for example, 

 at the stage of 25 or 30 somites) there develop a few direct dorso-lateral arteries 

 to the cardinal veins, and these arteries may lie opposite the somites instead of 

 between them. Such an artery, for example, is shown on the left side of figure 3 of 

 plate 3 This section is the next one below that of figure 2 of plate 3 in the series and 

 indeed the edge of this artery is shown in the latter section. Similar direct dorso- 

 lateral arteries to the cardinal veins are shown on both sides of figure 4 of plate 3. 

 This latter figure demonstrates that these dorso-lateral arteries are new vessels and 

 not remnants of the original dorsal arches. On the left side of figure 4 of plate 3 

 blood reaches the cardinal vein in three ways: from the aorta along the surface 

 of the cord, from the aorta along the primary dorsal arch, and from the aorta 

 through a dorso-lateral artery. It must also be brought out that these dorso- 

 lateral arteries to the cardinal vein are not the same as the direct lateral arteries 

 to the tubules of the pronephros and the metanephros, which develop later and 

 are quite differently placed, as can be seen in text-figure 8 from a chick of 35 

 somites. These dorso-lateral arteries to the cardinal veins are of importance in 

 connection with the extension of the cardinal veins caudalward and are very im- 

 portant in comparing the chick with a form like the pig, where the dorso-lateral 

 branches are more numerous. 



It may be well to enumerate here the different types of branches of the aorta 

 which may be found in the embryo from the standpoint of the structures they 

 supply: first, there are the arteries to the splanchnopleure; second, mesial branches 

 which connect the two aortse; third, lateral arteries to the somatopleure leading 

 to the umbilical veins; fourth, dorso-lateral arteries to the cardinal veins; fifth, 

 lateral arteries to the limb-buds; and sixth, lateral arteries to the nephritic tubules. 



