THE LATERAL MESODERM 201 



probably forms the germ cells in the sexually mature adult. The details of 

 this differentiation will be discussed later, and at this point we will simply 

 indicate that the germinal epithelium is probably induced by the Wolffian 

 duct and the notochord. In fact, transplantations of either of these latter 

 structures will cause a secondary germinal epithelium to develop from the 

 peritoneum of the amphibian embryo. 



The lateral mesoderm in the chick embryo extends into the extraembry- 

 onic areas, where the splanchnic layer becomes attached to the endoderm and 

 the somatic layer to the ectoderm. These double layers are sometimes termed 

 splanchnopleure and somatopleure, respectively. Thus, during the foldings 

 and growths involved in the formation of the extraembryonic structures, such 

 as the amnion and chorion, the mesoderm and ectoderm behave as a single 

 layer. Similarly, the mesoderm and endoderm remain attached to each other 

 while the allantois and yolk sac develop. Figure 103 illustrates the formation 

 of the amnion and the yolk sac. 



The importance of the mesodermal layer in the allantois and the yolk sac 

 lies in the fact that the blood vessels develop and grow in mesoderm. Thus 

 the vitelline vessels and the allantoic vessels, both of which are concerned 

 with the nutrition and respiration of the embryo, reside in the splanchnic 

 layer of the mesoderm. 



On the other hand, the somatic layer which covers the amnion forms 

 muscles which contract rhythmically to keep the amniotic fluid swirling 

 around the developing embryo. The amnion forms a firm cover, offering 

 mechanical protection to the embryo. 



Both the amnion and the allantois are purely extraembryonic structures 

 and are discarded at hatching. These structures thus represent adaptations to 

 the environment of the developing chick embryo. As we shall see in later 

 chapters, the functions of respiration, nutrition, excretion, and mechanical 

 protection are carried on by the extraembryonic membranes derived from the 

 somatopleure and splanchnopleure. 



General considerations 



In the past few chapters we have considered the development of the 

 nervous, circulatory, and digestive systems and some of the derivatives of 

 the mesoderm. Although there are still large gaps in our knowledge of the 

 causes underlying the development of many structures, it seems clear that 



