EM BR YO WI TH, EIGH T SEGMEN TS. 1 93 



a complete segment. The mesoderm does not extend across the median line, being 

 blocked by the notochord, nch. The somite is bounded mesially by the medullary 

 tube and notochord, dorsally by the ectoderm, ventrally by the aorta and entoderm. 

 Its cells are so arranged as to make a dorsal, a mesial, and a ventral wall, and 

 a core of cells more loosely grouped. Careful study of the segments in various 

 stages has led to the conclusion that the core belongs to the ventral wall. The 

 line of contact between the dorsal wall and the core is accordingly the potential 

 prolongation of the ccelom, and in certain embryos there is a ccelomatic space pres- 

 ent in the position indicated. The somite has a broader part toward the medullary 

 wall and a narrower part toward the main coelom. When a segment undergoes its 

 full development, the narrow part forms a separate structure, the nephrotome. 



Section through the Segmental Zones (Fig. 144). The medullary groove, Md, 

 is not closed, but is deep and narrow, its dorsal lips nearly in contact with one 

 another. The embryonic ectoderm, EC, is slightly thickened. The entoderm is 



Mes. Seg. z. Md 



Ent. Ao. nch. Cce. Ve. 



FIG. 144. SECTION OF A CHICK J^MBRYO WITH EIGHT SEGMENTS. TRANSVERSE SERIES 642, SECTION 267. 

 Ao, Dorsal aorta passing from the embryo to the area vasculosa. Cos, Beginning of coelomatic cavity. EC, 

 Ectoderm. Ent, Entoderm. Md, Medullary groove. Mes, Mesoderm. nch, Notochord. Seg. z, Seg- 

 mental zone of the mesoderm. Ve, Blood-vesseJ of the area vasculosa lying below the mesoderm proper. 

 X 100 diams. 



a thin layer which on one side, Ent, shows the beginning of the thickening char- 

 acteristic of the area opaca. The notochord, nch, is nearly circular in section and 

 is larger here than nearer the head. The dorsal aortae, Ao, have left their posi- 

 tion and are passing outward to ramify upon the area pellucida. It is thus evi- 

 dent that the distribution of the blood from the heart to the area vasculosa takes 

 place considerably caudad of the veins which collect the blood and return it to the 

 heart. The blood-vessels, Ao.Ve, lie between the mesoderm proper and the ento- 

 derm, and constitute with the associated blood-islands the angioblast. The situa- 

 tion of the angioblast in early stages is typical for all birds and also for mammals. 

 The mesoderm, Mes, has only irregular spaces, Coe, which by their expansion and 

 fusion will give rise to the continuous coelom. On either side of the medullary 

 groove, the mesoderm forms a thickened mass, the segmental zone, Seg.z, which 

 is markedly constricted where it joins the lateral mesoderm. Out of the constricted 

 area the nephrotomes are differentiated. The somites arise by transverse cleavage 

 of the segmental zone, each new somite being formed immediately caudad to the 

 last-formed somite. The so-called cleavage depends upon a great loosening of the 



