194 



STUDY OF YOUNG CHICK EMBRYOS. 



mesenchyma, as shown in figure 142, and not upon the development of an actual 

 fissure or cell-less space. If the series of sections be followed toward the tail, the 

 segmental zone will be found to fade out gradually. 



Section through the Open Medullary Groove (Fig. 145). As we have seen in 

 following the series of sections, the farther tailward we pass the less advanced is 

 the development. In the present section (Fig. 145), we find that only the notochord, 

 nch, is separated from its germ -layer. The three germ -layers, the ectoderm, EC, 



Aid. 



Ales. 



Ent. nch. Ve. 



FIG. 145. SECTION OF A CHICK EMBRYO WITH EIGHT SEGMENTS. TRANSVERSE SERIES 642, SECTION 314. 



EC, Ectoderm. Ent, Entoderm. Md, Medullary groove. - Mes, Mesoderm. nch, Notochord. Ve, Blood- 

 vessels. X loo diams. 



mesoderm, Mes, and entoderm, Ent, are merely laminae of undifferentiated cells, 

 the ectoderm alone showing a modification where it forms the wall of the medul- 

 lary groove, .Md. The angioblast or layer of blood-vessels, Ve, forms a well- 

 defined separate anlage, which is clearly distinct from both mesoderm'and entoderm. 

 The notochord, nch, is of large size, and in part occupies 'a notch on the .under 

 side of the medullary groove. A little farther caudad the notochord fuses with the 

 wall of the overlying medullary groove, and still farther on the united structures 

 fuse also with the entoderm (Fig. 146). 



Md 



Ales. 



Ent. Pr.S. 



FIG. 146. SECTION OF CHICK EMBRYO WITH EIGHT SEGMENTS. SERIES 642, SECTION 350. 

 EC, Ectoderm. Ent, Entoderm. Md, Medullary groove. Mes, Mesoderm. Pr.S, Primitive streak. X.ioo 



diams. 



Section through the Medullary Groove, near its Caudal End (Fig. 146). The 

 disposition of the parts is somewhat similar to that in figure 145, but the following 

 differences are to be noted: The medullary groove, Md, is deeper and more trough- 

 like in section and its floor merges into the primitive streak, Pr.S, or axial cord of 

 cells, which merges below with the entoderm and laterally with the mesoderm. The 

 fusion with all three germ-layers is the essential characteristic of the primitive 

 streak. The mesoderm, Mes, of the present section is voluminous, and shows in 



