38 



EARLY EMBRYOLOGY OF THE CHICK 



derm extending to either side between the ectoderm and ento- 

 derm. As would be expected from the method of origin, little 

 mesoderm appears in the mid-line except posterior to the primi- 

 tive streak. Immediately to either side of the mid-line the 

 mesoderm is markedly thicker than it is farther laterad (Fig. 

 IS, B). In whole-mounts the positions of the regional thicken- 



.^'^^^^^^ ^ 



plate 



pnmitive ^> 



streak ^ 



K....-rf 



pellucida^ 



primitive 

 streak 



CHICK OF ABOUT I4 HOURS. 



anterior horn of 

 mesoderm 



^ CHICK OF ABOUT 18 HOURS. 



proamnion 



dorsal mesoderm 



primitive streak 



\^ CHICK OF ABOUT 22 HOURS. 



Fig.! 2. — Schematic diagrams to show the extension of the mesoderm during 

 the latter part of the first day of incubation. Some of the more prominent 

 structural features of the embryos are drawn in lightly for orientation but the 

 ectoderm is supposed to be nearly transparent allowing the mesoderm to show 

 through. The areas into which the mesoderm has grown are indicated by 

 diagonal hatching. 



ings of the mesoderm are evidenced by the greater opacity they 

 impart to the embryo locally (Fig. 14). These thickened zones 

 of the mesoderm are the primordia of the dorsal mesodermic 

 plates. Because of the way in which they are later divided into 



