I \IHKYO OF SEVEN SEGMENTS 41 



portal, are the vitelline veins, just making their appearance at this stage. 

 The lips of the neural folds have met throughout the cranial two-thirds of 

 the embryo but have not fused. The neural tube, formed thus by the 

 closing of the ectodermal folds, is open at either end at the neuropores. 

 Cephalad, the neural tube has begun to expand to form the brain vesicles. 

 Of these only the fore-brain is prominent, and from it the optic vesicles 

 are budding out laterally. The paraxial mesoderm is divided by trans- 

 verse furrows into seven pairs of block-like primitive segments. Caudally, 

 between the segments and the primitive streak, there is the undifferen- 

 tiated mesoderm of the segmental zone, but new pairs of segments will 

 develop in this region. Looking through the open neural groove (rhom- 

 boidal sinus), one may see the notochord extending from the primitive 

 knot cephalad in the midplane until it is lost beneath the neural tube in 

 the region of the primitive segments. The primitive streak is still prominent 

 at the posterior end of the area pellucida, forming about one-fourth the 

 length of the embryo. 



TRANSVERSE SECTIONS 



Sections through the Primitive Streak and Knot. Conditions are essentially the 

 same as in the twenty -hour embryo (Fig. 31). 



Section through the Fifth Primitive Segment (Fig. 34). This general level is charac- 

 terized by the differentiation of the mesoderm, the approximation of the neural folds and 



Neural fold Neural groove 

 Ectoderm \ / Mesodermal segment 



Somatic mesoderm 



Splanchnic mesoderm / \ ^~^- Notochord 



Descending aorta Entoderm 



FIG. 34. Transverse section through the fifth pair of mesodermal segments of a twenty-five- 

 hour chick embryo. X 90. 



the presence of two vessels, the descending aortas, on each side between the mesodermal 

 segments and the entoderm. The neural folds are thick and the ectoderm is thickened 

 over the embryo. The notochord is a sharply denned oval mass of cells. The mesoder- 

 mal segments are somewhat triangular in outline and connected by the intermediate cell 

 mass, or nephrotome, with the lateral mesoderm. The lateral mesoderm is partially 

 divided by irregular flattened spaces into two layers, the dorsal of which is the somatic 

 layer, the ventral the splanchnic layer. Later, the spaces unite on either side to form the 

 ccdom, or primitive body cavity. In the area opaca, more laterad than is represented in 

 the figure, the entoderm is intimately associated with the coarsely granular yolk. Below 

 the splanchnic mesoderm, blood islands and primitive blood vessels are forming; this portion 

 of the area opaca is termed the area vasculosa. 



