192 



STUDY OF YOUNG CHICK EMBRYOS. 



than a separate structure. The mid-ventral wall or floor of the hind-brain is 

 relatively thin, a feature which marks the transition to the spinal cord, which al- 

 ways has a thin floor-plate. The notochord, nch, is large and transversely oval in 

 section. The two dorsal aortae, Ao.D, occupy the same relative positions as in the 

 previous sections. The ccelom, Coe, is a comparatively narrow fissure, but can be 

 followed laterally far out into the area op^aca. It is bounded above and below by 

 mesoderm, Msth, for the most part thin and of a loose texture; but on the lower 

 side in the embryonic region the mesoderm forms a, broad band, the cells of which 

 are densely packed. The thick mesodermic band is continuous with that which 

 forms the covering of the septum trans versum (Fig. 140, Msth). The morphological 

 significance of the band is undetermined. The mesenchyma, mes, occupies the 

 space between the hind-brain and ectoderm on the one hand and the aorta and 



EC. 



Som.m. 



Ent. 



Cos. Ao.D. nch. 



Mes. 



seg. Spl.m. 



FIG. 143. SECTION OF A CHICK EMBRYO WITH EIGHT SEGMENTS. TRANSVERSE SERIES 642, SECTION 180. 

 Ao.D, Dorsal aorta. Cos, Crelom. EC, Ectoderm. Ent, Entoderm. Md, Hind-brain. Mes, Mesoderm. nch, 

 Notochord. seg, Mesodermic segment or somite. Som.m, Somatic mesoderm. Spl.m, Splanchnic meso- 

 derm. X 100 diams. 



ccelom on the other. It consists of loosely scattered cells, connected with one 

 another by strands of protoplasm. It fuses at the proximal angle of the coelom with 

 the lining mesoderm thereof. Longitudinal sections demonstrate that the loose 

 mesenchyma occupies only the narrow space between two segments, and that it 

 fuses with the denser tissue of both adjacent segments. The intersegmental space 

 is not a cleft but a partition of loose mesenchyma. 



Sections through the Third Segment (Fig. 143). As the hind-brain ends at the 

 level of the fourth segment, the present section is near the transition from the 

 cephalic to the cervical region, segment 5 being the first cervical segment. Accord- 

 ingly we find that the medullary tube, Md, although not yet closed, has in cross- 

 section a form resembling that of the cord at this stage. The ectoderm, EC, runs' 

 from the lips of the medullary groove as a layer which is somewhat thickened over 

 the embryo, but becomes very thin over the area pellucida. The coelom, Coe, is of 

 small dimensions, although irregular clefts in the mesoderm of the germinative area 

 indicate its extension. The somatic mesoderm, Som.m, is a thin layer above the 

 ccelom; the splanchnic mesoderm is a much thicker layer, Spl.m, below the ccelom. 

 Both mesodermic layers extend beyond the coelom toward the medullary tube to 

 form the mesodermic somite, seg, which with its fellow of the opposite side constitutes 



