CHICK EMBRYO OF TWENTY-SEVEN SEGMENTS 6j 



vesicles. The lens of the eye has invaginated, pushing in the wall of the optic 

 vesicle and thus forming a double-walled structure, the optic cup. The audi- 

 tory placode has become a sac, the otocyst, which overlies the hind-brain opposite 

 the second branchial groove and is still connected with the outer ectoderm, cut 

 away in Fig. 55. The rhomboidal sinus is still open at the caudal end of the 

 neural tube (Fig. 54) . 



Digestive Canal. — In a reconstruction from the ventral side the digestive 

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

 seen in earlier stages; the mid-gut is without ventral wall and overlies the yolk. 

 A greater part of the mid-gut has been cut away to show the underlying struc- 

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

 evaginate into the tail-fold. The pharyngeal membrane now lies in a consider- 

 able cavity, the stomodceum, formed by the invaginated ectoderm. The median 

 ectodermal pouch next the brain-wall is known as Rathke's pocket and is the an- 

 lage of the anterior lobe of the hypophysis. The pharynx shows laterally three 

 out pocketings, of which the first is wing like and is the largest. These pharyngeal 

 pouches occur opposite the three branchial grooves and at these points entoderm 

 and ectoderm are in contact. Between them are developed the branchial arches, 

 in which course the paired aortic arches. Towards the fovea cardiaca the fore-gut 

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

 ventrally a bilobed structure, the anlage of the liver (Fig. 60). 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 grows out into the head-fold to form the fore-gut so 

 it grows into the tail-fold and 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. 



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 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 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 



