ORIGIN OF THE MESODERM, NOTOCHORD AND NEURAL TUBE 33 



The lips of the slit fuse, forming the primitive streak. The primitive 

 groove may be interpreted as a further futile attempt at invagination in 

 the region of the blastopore. The teachings of comparative embryology 

 support these conclusions, for the neurenteric canal arises at the cranial 

 end of the primitive streak, the anus at its caudal end, while the primary 

 germ layers fuse in its substance. All these relations exist at the blasto- 

 pore of the lower animals. 



FIG. 24. Diagram elucidating the formation of the primitive streak (Duval in Heisler). 

 The increasing size of the blastoderm during development is indicated by dotted circular 

 lines. The heavy line represents the crescentic groove from which the primitive streak arises 

 by the fusion of its edges. 



From the thickened ectoderm of the primitive. streak a proliferation 

 of cells takes place and there grows out laterally and caudally between the 

 ectoderm and entoderm a solid plate of mesoderm (Fig. 3 1 B, C} . This 

 soon splits into somatic and splanchnic layers (Fig. 34). An axial 

 growth, the head process, or notochordal plate, likewise extends forward from 

 the primitive knot (Figs. 25 and 30). This fuses at once with the ento- 

 derm and from its sides grow out lateral wings of mesoderm (Fig. 31 A). 



Neural plate Primitive knot 



Primitive pit Primitive streak Ectoderm 



\ Yolk 



Head process Entoderm Mesoderm 



Yolk 



FIG. 25. Median longitudial section of a chick embryo at the stage of the primitive streak and 



head process. X 100. 



Since the primitive streak and groove represent a modified blastopore, it 

 is evident that this cranial extension, the head process, corresponds to the 

 pouchlike invagination concerned in the formation of notochord and 

 mesoderm in reptiles. In birds the- fusion of the head process with the 

 entoderm, the relation of mesodermal sheets to it laterally, the formation 

 of the notochord from its tissue and the occasional traces in it of a cavity 

 continuous with the primitive pit (that is, a notochordal, or neurenteric 

 canal), all recall the conditions described for the less modified invagination 

 in reptiles. 



