38 



THE STUDY OF CHICK EMBRYOS 



midline, a depression in the ectoderm is the primitive groove. In this region there 

 is no Kne of demarcation between ectoderm and mesoderm. A transverse section 

 through the primitive knot (Fig. 315; guide Kne 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. i\ A; guide line A, Fig. 30), shows the thickening near the 

 midline 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 becomes 

 prominent at twenty hours the differ- 

 entiation of the germinal disc is rapid. 

 A curved fold, at first invol ving the 

 ectoderm and entoderm alone, is formed 





\lIead}old 



Ifead 

 process 



I'rimitive 

 A I'.gment 

 Primitive 

 knot 



I'rimitive 

 •■Ireak 



■> ■, Blood 

 "M island 



cephala d of the notochordal pro cess. 

 This is the head fold and is the aniage of 

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

 T he ectoderm has thickened on each 

 side of the mid-dorsal line, formin g the 

 neural folds. The groove between these 

 is the neural groove. The closure of this 

 groove will form the neural tube , the an- 

 lage of the central nervous syste m. The 

 notochord is now differentiated from the 

 mesoderm and may be seen in the mid- 

 dorsal line through the ectoderm. .In the 

 mesoderm lateral to the notochord a nd 

 cephalad to the primitive node, transverse furrows have differentiated two pairs 

 of block-like mesoderma l segment s, one j ncomplete cranially. As development 

 proceeds these increase in number, successive pairs being developed caudally. 

 They will be described in detail later. 



EMBBXO-QE_5E3?gN_SEG]VIENTS.Xi:::g ^TY-FIVE HOURS' INCUBAT ION-) 

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

 blood cells in the^au dal porti on of the area opaca. Initscramal portion isolated 

 gr oups of blood and blood vessel-forming cells are seen as blood islands. To- 



FiG. 32. — Surface view of a twenty-one 

 hour chick embryo, in which the head fold and 

 first two pairs of primitive mesodermal seg- 

 ments are present. The head process is seen 

 under the neural groove (after Duval). X 13. 



