278 STUDY OF YOUNG CHICK EMBRYOS. 



the small notochord which appears quite deeply stained, and therefore stands out 

 conspicuously from the very loose mesenchyma by which it is surrounded. It is 

 not until later stages that the mesenchymal cells begin to crowd around the noto- 

 chord to constitute the anlage of the future vertebral column. At the present 

 stage the differentiation of the axial skeleton around the notochord has not be- 

 gun. As regards the hind-brain, h. b, we know that at its sides it is already con- 

 siderably thickened, but at its dorsal wall it is quite thin and has already ex- 

 panded considerably, thus initiating the formation of the thin ependymal roof 

 of the fourth ventricle. On either side of the hind-brain appears a blood-vessel, 

 card, the anterior cardinal, which by transformation and migration is to lead to 

 the formation of the jugular veins of the adult. 



Section through the Aortic End of the Heart (Fig. 157). The cervical region of 

 the head and the tip end of the region of the fore-brain are cut separately. On 

 the lower side of the pharynx is attached the double heart- tube, of which the 

 endothelial portion, endo, is in actual contact with the thick entoderm, En, which 

 forms the floor of the pharynx. The heart-tube shows its bend toward the right of 

 the embryo. There is a considerable space between the endothelial heart and the 

 muscular heart, m. hi, and this space is almost wholly free of tissue, except in the 

 immediate neighborhood of the pharynx itself. Close to the connection of the 

 heart-tube with the pharyngeal floor there runs off on either side the membrane 

 of the amnion. Where it starts from the embryo the amnion has considerable 

 thickness and appears somewhat folded in the section; but as it turns to cover 

 the embryo it becomes very thin. It consists only of two very delicate layers, 

 mesodermic and ectodermic, both one cell thick. The two layers lie close to- 

 gether, but are easily distinguished. On the right-hand side of the embryo the 

 raphe of the amnion may be observed, raph, and in this section it is constituted 

 by only two strands of mesoderm which pass over from the amnion on to the 

 chorion, Cho, or membrana serosa, as it has been called by many embryologists. 

 The arrangement of the envelopes of the head is somewhat more complicated. 

 Underneath the right * of the section of the cervical portion of the head runs the 

 splanchnopleure, Spl, in which one can readily distinguish numerous sections of 

 blood-vessels, which, on the side toward the embryo, are covered by mesoderm, 

 and on the side away from the embryo are covered by entoderm. If we follow 

 along the splanchnopleure to a point near the section of the region of the fore- 

 brain, we find that it encounters a circle of ectoderm, EC, which surrounds that 

 portion of the head. When the splanchnopleure reaches this ectoderm, its two 

 layers divide or split apart. The mesoderm bends off to the right f and forms, 



* This means the left side of the embryo. f The right of the embryo, the left-hand side of the figure. 



