80 



THE EARLY DEVELOPMENT OF MAMMALS. 



stage the mesoderm has no distinct boundary against the underlying entoderm. It 

 is thickest in the neighborhood of the primitive streak and thins out from that in 

 all directions. It very early comprises two easily recognizable classes of cells. One 

 of these forms a more or less distinct layer next to the yolk, and so distributes 

 itself as to form a network of cavities of which these cells become the boundaries, 

 thus developing the first blood-vessels. The cells which form them constitute the 

 angioblast. A portion of the angioblastic cells comes to lie in the cavities of these 

 primitive blood-vessels and is transformed into the first red blood-corpuscles of the 

 embryo. The second class of cells constitutes the mesoderm proper^ and forms a 

 more continuous sheet of undifferentiated, somewhat closely compacted cells, ex- 

 tending out from the primitive streak and lying between the angioblast and the 

 ectoderm. 



The Expansion of the Mesoderm. After the- mesoderm is once formed as a dis- 

 tinct layer, it seems to have no longer any connection with the entoderm or ecto- 



A 



FIG. 43. THREE DIAGRAMS OF EMBRYONIC AREAS OF CHICKS TO SHOW THE GROWTH OF THE MESODERM. 



The mesoderm is indicated by vertical shading, the area opaca by horizontal shading. A.o, Area opaca. A.p, 



Area pellucida. mes, Mesoderm. pr, Primitive streak. (After Duval.) 



derm, except in the axial line. Its further expansion is due to the proliferation of 

 its own cells. During this early expansion the mesoderm assumes in all amniota a 

 definite and characteristic series of outlines. It is at first pear-shaped (Fig. 43, A), 

 the anterior end being pointed. It extends a short distance only in front of the 

 primitive streak and is widest a little distance behind the area pellucida (Ap]. (For 

 a description of the area pellucida see Chapter V.) The condition in the chick at 

 about the twentieth hour of incubation is indicated by figure 43, B, drawn on the 

 same scale as A, and at the close of the first day by figure 43, C. In the last stage 

 figured it will be noticed that the mesoderm is expanding unequally in front, hav- 

 ing sent out two lateral wings which leave a median space between them without meso- 

 derm. These wings continue their growth, and finally meet in front, so that in the 

 anterior part of the area pellucida there is a small tract without any mesoderm, 

 although it is completely enclosed by mesoderm. This tract is the pro-amnion. The 



