EMBRYO OF TWENTY-SEVEN SEGMENTS 59 



into the mid-gut there is budded off ventrally a bilobed structure, the anlage 

 of the liver (Figs. 57 and 63). It lies between the vitelline veins, and in its 

 later development the veins are broken up into the sinusoids, or blood 

 spaces of the liver. 



Just as the entoderm participates in the head fold to form the fore- 

 gut, so in the tail fold it forms the hind-gut. This at once gives rise to a 

 tubular outgrowth which becomes the allantois, one of the fetal membranes 

 to be described later (Fig. 70). 



Blood Vascular System. The tubular heart is flexed in the form of a 

 letter S, when seen from the ventral side (Fig. 57). Four regions may 

 be distinguished: (i) the sinus venosus, into which the veins open; (2) a 

 dilated dorsal chamber, the atrium; (3) a tubular ventral portion flexed 

 in the form of a U, of which the left limb is the ventricle, the right limb (4) 

 the bulbus cordis. From the bulbus is given off the ventral aorta. There 

 are now developed three pairs of aortic arches which open into the paired 

 descending aortae. The first aortic arch passes cranial to the first pharyn- 

 geal pouch and is the primitive arch seen in the thirty-eight-hour embryo. 

 The second and third arches course on either side of the second pharyngeal 

 pouch. They are developed by the enlargement of channels in primitive 

 capillary networks between ventral and descending aortae. Opposite 

 the sinus venosus, the paired aortic trunks fuse to form the single dorsal 

 aorta which extends as far back as the fifteenth pair of primitive segments. 

 At this point the aortae again separate, and, opposite the twentieth 

 segments, each connects with the trunk of a vitelline artery which was 

 developed in the vascular area and conveys the blood to it (Fig. 57). 

 Caudal to the vitelline arteries the dorsal aortas rapidly decrease in size 

 and soon end. 



As in the previous stage, the blood is-conveyed from the vascular area to 

 the heart by the vitelline veins, now two large trunks. In the body of the 

 embryo there have developed two pairs of veins. In the head have 

 appeared the anterior cardinal veins, already of large size and lying lateral 

 to the ventral region of the brain vesicles (Fig. 60). Caudal to the atrium 

 of the heart, two small posterior cardinal veins are developed. They 

 lie in the mesenchyma of the somatopleure, laterad in position (Fig. 63). 

 Opposite the sinus venosus the anterior and posterior cardinal veins of each 

 side unite and form the common cardinal veins (ducts of Cuvier) which 

 open into the dorsal wall of the sinus venosus (Fig. 57). The primitive 

 veins are thus paired like the arteries, and like them develop by the en- 

 largement of channels in a network of capillaries. 



The following series of transverse sections from an embryo of this 

 stage shows the more important structures. The approximate plane 

 and level of each section may be ascertained by referring to Figs. 56 and 57. 



