252 



EMBRYOLOGY 



BLASTODERM 



OVERGROWTH 



Fig. 164. The relation of the grow- 

 ing chick embryo to its source of nutri- 

 ments. During very early development 

 the blastoderm is in direct contact with 

 the yolk and the products of digestion 

 diffuse into the cells. With a larger mass 

 of cells, as in the embryo, blood vessels 

 develop which convey the digested prod- 

 ucts to the embryo. Finally in the later 

 stages of development the yolk is en- 

 closed in a sac composed of endoderm 

 and mesoderm. The endoderm digests the 

 yolk and the blood vessels transport sub- 

 stances to the growing chick. 



ARTERIES 



can be used by the growing cells (Fig. 164). As the embryo gets larger, how- 

 ever, the endoderm incorporates this yolk into a sac — the yolk sac. The blood 

 vessels — the vitelline veins and arteries — then serve to transport the materials 

 from the yolk to the embryo. The circulation carries substances to the growing 

 parts of the chick embryo, where they are utilized for the synthesis of new 

 proteins. The yolk sac, therefore, is the embryonic digestive system. In the 

 yolk-sac membrane the enzymes break up the yolk into diffusible products, 

 which go through the blood vessels to the heart and thence to the rest of the 

 embryo. The yolk thus provides the raw materials for growth — proteins, fats, 

 and to some extent, carbohydrates. The composition of the yolk is discussed 

 in Chapter 15. 



