ORIGIN OF THE MESODERM, NOTOCHORD AND NEURAL TUBE 



33 



knot essentially as in birds (Figs. 26 A and 28). Similarly from the keel-like 

 ectodermal thickening of the primitive streak, mesoderm grows out laterally and 

 _ caudally, an d from the primitive knot it is continued cranially as the head process. 

 All three primary germ layers fuse in the primitive streak and knot, this condition 



Head fold 

 Ecto- 

 derm 



Neural plate Primitive knot 



Primitive pit Primitive streak 



Entoderm 



Mesoderm 



Yolk 



Fig. 25. — Median longitudinal section of a chick embryo at the stage of the primitive streak and head 



process. X 100. 



being known in man. The head process of many mammahan embryos contains a 

 cavity {notochordal canal) , which in some cases is of considerable size, opening at 

 the primitive pit. As in reptiles, the floor of this cavity fuses with the entoderm 

 and the two rupture and disappear. A still persistent portion of the floor is shown 

 in Fig. 27. Thus a neurenteric 

 canal, later enclosed by the 

 neural folds, puts the dorsal 

 surface of the blastoderm into 

 communication with the enteric 

 cavity beneath the entoderm 

 (Figs. 77 and 78). The roof of 

 the head process or notochordal 

 canal is for a time continuous 

 with the mesoderm and ento- 

 derm (compare these relations 

 in reptiles. Fig. 22), but it event- 

 ually becomes the notochord. 



The extent of mesoderm 

 in rabbit embryos is shown in 

 Fig. 28. Cranial to the primi- 

 tive node the notochord is differentiated in the midline, the mesoderm being 

 divided into two wings. The mesoderm rapidly grows around the wall of the 

 blastodermic vesicle until it finaUy surrounds it and the two wings fuse ven- 

 trally (Fig. 29) . The single sheet of mesoderm soon sphts into two layers, the 

 cavity between being the ccelom or bod x cavity. The outer mesodermal layer 

 (somatic), with the ectoderm, forms the somgto^leu?x_or body wall, the inner 



3 



Fig. 26. — The primitive streak of pig embryos (Kei- 

 bel). X 20. A, Embryo with primitive streak and prim- 

 itive knot; B, a later embryo in which the neural groove is 

 also present, cephalad in position. 



