470 Anatomy and Devolopinont of Veins of Chelonia 



point where they open into the hepatic vein, and in a l-t mm. emhryo they 

 no longer connect with the right hepatic except throngh the general sinu- 

 soidal network of the liver. In tlie 15 mm. stage the derivatives of the 

 omphalo-mesenteric veins have practically assumed their adult relations. 



The Umbilical A'eins. The Formation of the Eight axd Left 

 Abdominal Yeixs axd the Left Hepatic Veix. 



It is not possil)le from the material at hand to determine anything 

 definite about the origin of the umbilical veins in turtles. Dr. Frederic 

 T. Lewis, who has made a very careful study of the early development 

 of the umbilical veins in the rabbit, has very kindly sent the writer a 

 number of drawings and .sketches (not yet published) which undoubtedly 

 show that, in the rabbit at least, the umbilical veins appear earlier than 

 the postcardinals and arise in part from the vitelline veins and in part 

 from the intersegmental arteries, thus : " The vitelline vein sends sprouts 

 forward and backward in the somatopleure. The forward branch be- 

 comes the anterior cardinal vein: the posterior branch, incorporating in 

 its progress prolongations of the intersegmental arteries, becomes the 

 umbilical vein.'' (Lewis : Am. Jour. Anat., 1904, Vol. Ill, p. XII.) 



In the 7.4 mm. embryo of the turtle, as we have already seen, the 

 postcardinals and umbilicals are both present. The umbilicals are, how- 

 ever, much smaller than the postcardinals and are connected with them, 

 especially near their cardiac ends, and through the region of the anterior 

 limb buds, by a large numljer of cross branches. Caudally, the umbilical 

 veins cannot be followed with certainty. 



In the 9 mm.^ embryo, however, they become very large and may easily 

 be followed along their entire length. At this stage (9 mm.) they are 

 of nearly equal size. They arise in the allantois and extend along the 

 body wall near the somato-amniotic angle and open into the sinus 

 venosus close to, or in common with, the omphalo-mesenteric vein of the 

 same side. In most embryos of this age the right umbilical vein appears 

 to open into the right omphalo-mesenteric vein, while the left opens 

 into the sinus venosus some distance from the omphalo-mesen- 

 teric vein of that side. Both umlulicals are still somewhat connected 

 with the postcardinals especially through the limb anlages. . 

 Umbilical veins of a 10 mm. emhrijo. — The umbilical veins are now 

 increasing very rapidly in size and importance. They begin, at about 

 this time, a series of changes which is destined to play an important role 

 in the development of the portal system of the liver. This is brought 

 about chiefly by the separation of the pericardial cavity. 



