EMBRYO WITH TWENTY-EIGHT SEGMENTS. 199 



thirds 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 0f the embryo and 

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

 the anlage of the spinal cord. At the side of the third vesicle we can see an open 

 pit, the anlage of the inner ear or otocyst, Ot. On the side of the neck between 

 the third cerebral vesicle and the heart there are three external depressions which 

 bound the first and second branchial arches, i, 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 branchial arch the 

 outline of the embryo makes a depression which marks the position of the develop- 

 ing, oral cavity. The heart is a large tube, Ht, in a still larger pericardial cavity 

 (ccelom), the membranous covering of which is somatopleuric. 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 

 aorta which sends off the two vitelline arteries, A.m. On either side of the med- 

 ullary canal can be seen the primitive segments, Seg. The first of these which 

 is distinct lies close behind the otocyst. At the. posterior end of the embryo addi- 

 tional segments are still forming, and the precise number of segments varies from 

 embryo to embryo. The medullary canal, Md, is closed, but beyond its extreme 

 limit traces of the primitive groove, Pr.g, can still be seen. The network of 

 blood-vessels over the area vasculosa is very distinct and characteristic. The net- 

 work, however, does not yet extend into the body of the embryo proper. The 

 limit of the body of the embryo is suggested by the darker tissue, Seg.z, surround- 

 ing the spinal cord, Md, on either side. About the hinder end of the embryo, both 

 iri the pellucida and in the opaca, appear, a number of small spots, the blood- 

 islands, bl.is, many of which have in the fresh specimen a reddish color. In hard- 

 ened specimens the opacity of the blood-islands renders them conspicuous, espe- 

 cially in the area pellucida. 



Embryo Chick with Twenty-eight Segments. 



The Study of Transverse Sections. A series of figures from transverse sec- 

 tions of an embryo of this stage is herewith presented. They have been selected 

 so as to show the principal typical structures. The position of the sections can 

 be followed more easily by comparing each transverse section with figure 166, 

 to determine its place and the organs through which it must pass. 



Section through the Right Auditory Imagination (Fig. 151). Owing to the 

 curvature of the neck-bend, the section of the head is not symmetrical. It passes 

 through both the hind-brain, h.b, and the fore-brain, f.b. Underneath the former 

 appears- a small structure, nch, the notochord, and on one side can be seen the 

 auditory invagination, O/, which is formed wholly by the locally thickened ectoderm, 



