280 STUDY OF YOUNG CJfJCK EMBRYOS. 



ectoderm by following it through in the se'ries of sections, for he will then find 

 that it becomes continuous in other regions, on the one hand, with the ectoderm 

 of the true amnion, and, on the other, with the epidermis of the body proper. In 

 the cervical region we have a transverse section of the lower portion of the 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 segment, Seg. Just below the seg- 

 ments is the 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 prolonga- 

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

 the third gill pouch. 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 entoderm, 

 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 ectoderm, and 

 the cells of the two germ-layers have there united, without distinguishable boun- 

 dary being kept between the layers. The fused ectoderm and entoderm consti- 

 tute the closing plate of the gill cleft, and such a plate is formed at the tip of every 

 gill pouch. On either side of the ventral surface of the pharynx appears the sec- 

 tion of the second aortic arch, Ao. 2. By following along through a few sections 

 (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 con- 

 tinuation 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, /. 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. 158). 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. 157), but there are many important differences of detail. We 

 are now in the region of the spinal cord, Sp. c, proper, 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- 



