34 



CLEAVAGE AND THE GERM LAYERS 



Mammals. On the blastoderm of mammals appear a primitive streak 

 and 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, and from the primitive knot there is continued 

 forward a head process. All three primary germ layers fuse in the primitive 

 knot, this condition being known in man. The head process of many 

 mammalian 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 notochordal canal, later enclosed by the neural folds, and then 

 known as the neurenteric canal, puts the dorsal surface of the blastoderm 



FIG. 26. Early blastoderms of pig embryos (Keibel). X 20. A, Embryo with primi- 

 tive streak and primitive knot; B, a later embryo in which the neural groove is also present, 

 cephalad in position. 



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 closely associated with the mesoderm and entoderm (compare these 

 relations in reptiles, Fig. 22), but it eventually becomes the notochord. 



The extent of mesoderm in rabbit embryos is shown in Fig. 28. 

 Cranial to the primitive knot the notochord is differentiated in the mid- 

 plane, and the mesoderm extends laterally as two wings. The mesoderm 

 rapidly grows around the wall of the blastodermic vesicle until it finally 

 surrounds it and the two wings fuse ventrally (Fig. 29) . The single sheet 

 of mesoderm soon splits into two layers, the cavity between being the 

 ccdom, or body cavity. The outer mesodermal layer (somatic), with the 

 ectoderm, forms the somatopleure, or body wall; the inner splanchnic 



