THE EARLY HISTORY OF THE MESODERM. 75 



tion 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. 29, 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 

 Fig. 29, B, drawn on the same scale as A , and at the close of the first day by Fig. 

 29, C. In the last stage figured it will be noticed that the mesoderm is expand- 

 ing unequally in front, having sent two lateral wings which leave a median space 

 between them without mesoderm. 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 meso- 



mes ■ 



Fig. 29. — 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. 



^./, Area pellucida. mes, Mesoderm, /r, Primitive streak. — {After Duval.) 



derm. This tract is the pro-amnion. The actual expanding edge of the meso- 

 derm is quite irregular. The regularity shown in Fig. 29 is entirely diagram- 

 matic. 



The extent of the growth of the mesoderm over the extra-embryonic region 

 of the mammalian blastodermic vesicle is very variable. Usually it extends 

 completely around the vesicle, but in some cases, as in the rabbit, only part way 

 (compare page 65, ante). 



The Origin of the Coelom. — The next step in the differentiation of the middle 

 germ-layer is the appearance of two slit-like cavities in it, one on each side. 

 These cavities do not extend across the median line, for when they appear there 

 is no mesoderm in the median line of the embryo. The coelom is the anlage of 

 the body-cavity, and in part persists in the adult as the pericardial, pleural, and 



