EMBRYO OF SEVEN SEGMENTS 



39 



Head fold 



A transverse section through the primitive streak at twenty hours 

 (see guide line C, Fig. 30) shows the three germ layers distinct laterally 

 (Fig. 31 C). In the midline, a depression in the ectoderm is the primitive 

 groove. In this region there is no line of demarcation between ectoderm 

 and mesoderm. A transverse section through the primitive knot (Fig. 

 31 B\ guide line B, Fig. 30) shows the three germ layers intimately fused 

 (cf. Fig. 51). There is a marked proliferation of cells, which are growing 

 cephalad to form the notochordal plate (head process) (cf. Fig. 25). 



A transverse section through the notochordal plate, just beginning 

 to form at this stage (Fig. 31 A ; guide 

 line A, Fig. 30) shows the thickening 

 in the midplane which will separate 

 from the lateral mesoderm and form 

 the notochord. It is fused with the 

 entoderm but not with the ectoderm. 



After the notochordal plate be- 

 comes prominent at twenty hours, 

 the differentiation of the blastoderm 

 is rapid. A curved fold, at first in- 

 volving the ectoderm and entoderm 

 alone, is formed cephalad of the 

 notochordal process. This is the 

 head fold, and is the anlage of the 

 head of the embryo (Figs. 25 and 32). 

 The ectoderm has thickened on each 

 side of the mid-dorsal line, forming 

 the neural folds. The groove between 

 these is the neural groove. The 

 closure of this groove will later form 

 the neural tube, the anlage of the 

 central nervous system. The notochord is now differentiated from 

 the mesoderm and may be seen in the median plane through the 

 ectoderm. In the mesoderm, lateral to the notochord and cephalad 

 of the primitive knot, transverse furrows have differentiated two pairs 

 of block-like mesodermal segments, one incomplete cranially. As develop- 

 ment proceeds these increase in number, successive pairs being developed 

 caudally. They will be described in detail later (p. 53). 



EMBRYO OF SEVEN SEGMENTS (TWENTY-FIVE HOURS' INCUBATION) 



In this embryo (Fig. 33) there is a prominent network of blood vessels 

 and blood cells in the caudal portion of the area opaca. In its cranial 

 portion isolated groups of blood and blood vessel-forming cells are seen 



FIG. 32. Surface view of a twenty-one 

 hour chick embryo, in which the head fold 

 and first two pairs of primitive mesodermal 

 segments are present. The head process 

 is seen through the neural groove (after 

 Duval). X 13. 



