EMBRYO WITH TWENTY-FOUR SEGMENTS. 273 



center of the area vaseulosa appears the embryo, the head end of which is twisted 

 over so that the left side of the head lies against the yolk. This twisting of the 

 neck and head so that they become asymmetrical in position is very characteristic 

 of birds. Below the head and somewhat to the right may be seen the tubular 

 heart, Hi, which, in the fresh specimen, pulsates regularly. Around the area 

 pellucida comes the dark area opaca, in which we readily distinguish the outer 

 boundary, or terminal sinus, of the area vaseulosa. In this there is already a 

 well-developed network of blood-vessels through which the blood is circulating, 

 being driven by the heart. The blood moves out from the embryo by two large 

 vessels, A. vi, which lie symmetrically, the vitelline or omphalo-mesaraic ar- 

 teries. These arteries arise from the dorsal aorta of the embryo and pass out to 

 the area vaseulosa, over which they ramify. The blood returns to the heart by 

 means of the omphalo-mesaraic veins, of which the anterior branches * are alone 

 clearly differentiated at this stage. The general form of the embryo is indicated 

 by figure 152. In the region of the head we notice the very well-marked head- 

 bend which is established in the region of the mid-brain, M. b. The medullary 

 tube in the region of the head is very much enlarged and is divided into three 

 well-marked primary cerebral vesicles. The first of these is quite large, and at 

 its side lies the anlage of the eye, Op, in the center of which one readily distin- 

 guishes the commencement of the lens. The second cerebral vesicle is much 

 smaller than the first in every dimension. It occupies the region of the head- 

 bend and is separated from the first vesicle by a constriction, and from the third 

 vesicle by another constriction. The third vesicle in length more than equals 

 the first and second combined, and at its widest part is nearly equal in diameter 

 to the second vesicle. It tapers out toward the caudal end of the embryo and 

 passes over into the much smaller portion of the medullary canal, which repre- 

 sents, the anlage of the spinal cord. At the side of the third vesicle we can see 

 the beginning of the formation of the ear or otocyst, Ot. On the side of the neck 

 between the third cerebral vesicle and the heart there are three external depres- 

 sions which bound the first and second branchial arches, 1,2, of the embryo. 

 Behind each arch the depression marks the site of a gill cleft. The first is the 

 longer, the second the shorter. Between the projecting head and the first bran- 

 chial arch the outline of the embryo makes a depression, which marks the posi- 

 tion of the developing oral cavity. The heart is a large tube, Hi. The omphalo- 

 mesaraic veins join the venous or posterior end of the heart. The heart is very 

 much bent ; its anterior end turns toward the gill clefts and there gives off the 

 primitive aortic branches, which finally join again so as to form the median dorsal 



*The vessels do not appear in the figure. 



