EMBRYO WITH TWENTY -EIGHT SEGMENTS. 



203 



by the second aortic arch, near which appears an accumulation of more deeply 

 colored cells, cl.II, part of the entodermal wall of the second gill-pouch. Between 

 the pharynx and the hind-brain we have a round section of 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 notochord to constitute the 

 anlage of the future vertebral column. At the present stage the differentiation of 

 the axial skeleton around the notochord has not begun. As. regards the hind-brain, 

 h.b, we observe that its sides are already considerably thickened, but its dorsal 

 wall is quite thin and has already expanded 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 transforma- 

 tion and migration is to lead to the formation of the jugular veins of the adult. 



Section through the Aortic End of the Heart (Fig. 155). 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.ht, 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, meso- 

 dermic and entodermic, both one cell thick. The two layers lie close together, 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 arrange- 

 ment of the envelopes of the head is somewhat more complicated. Underneath the 

 left side 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 toward the right* side of the embryo and forms, together with 

 a portion of the ectoderm, a part of the true amnion, Am', of the head. The 



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



