EMBRYO OF TWENTY-SEVEN SEGMENTS 57 



Digestive Canal (Fig. 57).— In a reconstruction from the ventral side, the 

 digestive canal shows differentiation into three regions. Of these, the fore-gut 

 has been seen in earUer stages. A greater part of the mid-gut has been cut away 

 to show the underlying structures ; it is without a ventral wall and overhes the yolk. 

 Caudad, a small fovea leads into the hind-gut which is just beginning to evag- 

 inate into the tail fold. The pharyngeal membrane now hes in a considerable 

 cavity, the ^iatiwdiSMM) formed by the invaginated ectoderm. The median ecto- 

 dermal pouch next the brain wall is known as Rathke's pocket and is the anlage 

 of the anterior lobe of the hypophysis. The pharynx shows laterally three out- 

 pocketings, of which the first is wing-hke and is the largest. These pharyngeal 

 pouches occur opposite the three branchial grooves and here entoderm and ecto- 

 derm are in contact, forming the closing plates. At about this stage the first 

 closing plate ruptures, thereby forming a free opening, or branchial cleft, into the 

 pharynx. Between the pouches are developed the branchial arches, in which 

 course the paired aortic arches. Towards the fovea cardiaca the fore-gut is flat- 

 tened laterally and before it opens out into the mid-gut there is budded off ven- 

 trally a bilobed structure, the anlage of the liver (Figs. 57 and 63). It lies be- 

 tween the vitelline veins and in its later development the veins are broken up into 

 the sinusoids or blood spaces of the fiver. 



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. Four regions may be distinguished: (1) 

 the sinus venosus, into which open the veins; (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 fimb (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 aortse. The first aortic arch passes 

 cranial to the first pharyngeal pouch and is the primitive arch seen in the thirty- 

 six-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 aortas. Op- 

 posite 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 



