THE ORIGIN OF THE MIDDLE GERM LAYER 45 



After the notochordal plate becomes prominent at twenty hours the dif- 

 ferentiation of the germinal area is rapid. A curved fold, involving the three 

 layers of the germinal area, is formed cephalad to the notochordal process. This 

 is the head-fold and is the anlage of the head of the embryo (Fig. 27). The ecto- 

 derm 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 

 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 mid-dorsal line 

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

 to the primitive node, transverse furrows have differentiated a pair of mesodermal 

 segments. As development proceeds these increase in number, successive pairs 

 being developed caudally. They will be described in detail later. 



To sum up, in the chick the mesoderm appears with the formation of the 

 primitive streak. It originates from the primitive streak and node and spreads 

 in all directions between the other germ layers as an undivided plate of cells. 

 It grows cephalad in the midline as the notochordal process or plate from which 

 the notochord is developed. 



As the mesoderm is derived from the entoderm in Amphioxus, its origin is generally- 

 regarded as entodermal in birds and mammals. This would certainly be the case if we interpret 

 the notochordal process and entoderm as formed by a process of gastrulation. Keibel (in 

 Keibel and Mall, vol. I), however, holds that the mesoderm and notochordal plate are derived 

 from the ectoderm, and that any relation which they bear to the entoderm is of secondary origin. 



The Origin of the Mesoderm in Mammals. — As we have seen, the primitive 

 streak is formed on the surface of the germinal area in mammalian embryos as 

 in the chick. It has been described as due to a keel-like thickening of the ecto- 

 derm, and the knob-like mass of cells at its cephalic end, the primitive node, is 

 the first to appear. The mesoderm is formed precisely as in the chick, growing 

 out in all directions from the primitive streak and node between the other two 

 layers. Its extent in rabbit embryos is shown in Fig. 29 A and B. Cranial 

 to the primitive node the notochord is differentiated in the midline, the meso- 

 derm being divided into two wings. The mesoderm rapidly grows round the 

 wall of the blastodermic vesicle until it finally surrounds it and the two wings fuse 

 ventrally (Fig. 30 A and B). The single sheet of mesoderm soon splits into two, 

 the cavity between being the cozlom or body cavity. The outer mesodermal layer 

 (somatic), with the ectoderm, forms the somatopleure or body wall, the inner 

 splanchnic layer, with the entoderm, forms the intestinal wall or splanchnopleure. 

 The neural tube having in the meantime been formed from the neural folds of the 



