EMBRYO WITH TWENTY-EIGHT SEGMENTS. 205 



hind-brain, Md, corresponding to the part of the future medulla oblongata near 

 its junction to the spinal cord. Underneath it is the section of the notochord, nch, 

 and on either side sections of a secondary somite, Seg. Just below each somite 

 is a' cardinal vein, Ve, and below the vein, but nearer to the median line, lies 

 the dorsal aorta, Ao.D. The pharynx expands on each side; the prolongation on 

 the left of the embryo is the second gill-pouch, cl.II, that on the right is the 

 third gill-pouch, cl.III. The pharynx itself is lined by entoderm, En, which is 

 very thin in the median dorsal line, but immediately below the dorsal aortae it 

 thickens abruptly and continues as a quite thick layer on to the ventral side. In 

 the median ventral line it forms a deep groove, and in the walls of this groove we 

 find that the nuclei are. not distributed through the whole thickness of the ento- 

 derm, but occupy chiefly its outer or basal portions, so that the portion of the 

 layer next the cavity of the groove is formed almost wholly of protoplasm. At the 

 tip of the gill-pouch the entoderm has come into actual contact with the ecto- 

 derm, and the cells of the two germ-layers have there united, without distin- 

 guishable boundary being kept between the layers. The fused ectoderm and ent 

 derm constitute the closing plate of the gill-cleft, and such a plate is formed at the 

 tip of every gill-pouch. On the left side of the ventral surface of the pharyn/ 

 pears the section of the second aortic arch, A 0.2. Opposite but higher up is 

 the section of the right third aortic arch. By following along through a few sec- 

 tions (in the series here studied, from four to six) the junction of these arches 

 with the endothelial tube of the heart may be observed. The student should verify 

 this connection and satisfy himself that the endothelium of the blood-vessels is a 

 continuation of the endothelium of the heart. This fact is of great morphological 

 and physiological importance. Of the section of the region of the fore-brain little 

 need be said. The ectoderm has begun to thicken somewhat. The walls of the 

 fore-brain, f.b, itself have not begun to show any differentiation into layers. There 

 is a considerable development of mesenchyma between the brain and the superficial 

 ectoderm. 



Section through the Venous End of the Heart (Fig. 156). We have now passed 

 in our series beyond the level of the head, so that no part of that is included in 

 the section. The general topography of the part is similar to that of the preced- 

 ing section (Fig. 155), but there are many important differences of detail. We 

 are now in the region of the spinal cord, proper, Sp.c, which here offers to us its 

 characteristic early embryonic form. It is oval in section, -its. walls are thickened 

 on each side, but are thinned on the dorsal side, where they constitute the deck- 

 plate, and on the ventral side, where they form the floor-plate; the cavity is narrow 

 and slit-like. The no f ochord ' close under the ventral side of the medullary 

 tube and below it i^ tb r median dorsal aorta, Ao, a single and very large vessel, 

 which is formed by f i < union of . v the two dorsal aortae shown in figure 157, 

 Ao.D. Immediately below in*, aorta f,\!o,vs the pharynx, Ph, which is no v more 

 rounded in form and does not e.-iend ar laterally. Its entodermal lining is mod- 



