THE ORIGIN OF THE MIDDLE GERM LAYER 



43 



Head process 



Neural 

 groove 



A 



Primitive 

 node 



Primitive 

 groove 



Area opaca 



streak 

 Blood island 



FIG. 26. Dorsal surface view of a twenty-hour chick em- 

 bryo showing primitive streak and extent of mesoderm (after 

 Duval) . The lines A , B, and C indicate the levels of the cor- 

 responding sections shown in Fig. 28. 



chordate body is developed. 

 In Amphibia from the dor- 

 sal plate of entoderm the 

 mesodermal diverticula 

 grow out as solid plates 

 between ectoderm and en- 

 toderm. Later, these plates 

 split into two layers and 

 the cavity so formed gives 

 rise to the ccelom. 



Origin of the Meso- 

 derm in Chick Embryos. 

 If we examine a chick em- 

 bryo of twenty hours' in- 

 cubation (Fig. 26), it will 

 be seen that the primitive 

 streak is formed as a linear 

 opacity near the posterior 



border of the germinal area. Over a somewhat pear-shaped clear area the yolk 

 has been dissolved away from the overlying entoderm. This area, from its 



appearance, is termed the area pellucida. 

 It is surrounded by the darker and more 

 granular area opaca. Whether or not the 

 primitive streak represents the fused lips 

 of the blastopore, it is certain that it' 

 represents the point of origin for the 

 middle germ layer. It also indicates 

 the future longitudinal axis of the em- 

 bryo. Proliferation of cells takes place 

 here between ectoderm and entoderm 

 and there grows out laterally and caud- 

 ally between these layers a solid plate 

 of mesoderm, as in amphibia. The 

 shaded area in Fig. 26 shows the extent 

 FIG. 27. Surface view of a twenty-one- of the mesoderm. It extends at first 



hour chick embryo, in which the head-fold and -i, , , , ,, . ... 



( . .' more rapidly caudad to the primitive 



first pair of primitive mesodermal segments J 



are present (after Duval). streak, at the cranial end of which 



Blood island 



