274 STUDY OF YOUNG ClflCK EMBRYOS. 



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

 dullary 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 cf 

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

 network, 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 

 in 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 

 hardened specimens the opacity of the blood-islands renders them conspicuous, 

 especially 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 Invagination (Fig. 153). Owing to a 

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

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

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

 the auditory invagination, Ot, which is formed wholly by the locally thickened 

 ectoderm, which is elsewhere quite thin. The ectoderm, EC, covering the dorsal 

 side of the hind-brain is very thin, but the portion in front of the auditory invagi- 

 nation is somewhat thicker. The ectoderm of the invagination is very much 

 thickened and contains numerous somewhat crowded nuclei at all levels. These 

 nuclei are rounded in form and have one or two very distinct nucleoli. On the 

 posterior side of the otocyst there is very little mesoderm; on the anterior side, 

 much more. Between the developing otocyst and the notochord there is a blood- 

 vessel, ve, with merely endothelial walls, a branch of the cardinal vein. Between 

 the hind-brain and fore-brain near the notochord, the two aortae Ao, are cut. In 

 their interior there can usually be seen a certain number of nucleated cells vary- 

 ing somewhat in size and appearance, but generally having a rounded form with 

 distinct outline and a well-defined nucleolated nucleus. 



Section through the Left Auditory Invagination (Fig. 154). Owing to the 



