RAW MATERIALS FOR GROWTH 251 



tions. In this respect they would resemble the virus nucleoproteins because 

 once a cell is infected with the virus, the virus reproduces itself and each 

 cell which divides contains the virus. The virus spreads. In the same way, an 

 embryonic cell is "infected" with a substance causing differentiation. This 

 "infection" is a result of treating the cell with an inducing agent. Once "in- 

 fected," the cell divides and the substance causing differentiation is self-du- 

 plicated in the cytoplasm. Just as there are many types of viruses producing 

 different effects on cells, so we may assume many types of differentiation 

 substances producing the various types of cells in the embryo. 



Raw materials for growth 



There are still two important problems. In the course of growth and re- 

 duplication, the embryo needs substrates; it needs raw materials for syntheses. 

 That is one factor. We have also pointed out that these self-duplicating units 

 require a source of energy. In addition to the self-duplicating reaction the cell 

 must produce the energy requirement by means of oxidations. Otherwise 

 synthetic reactions would go in the reverse direction, and the self-duplicating 

 compounds would break down. 



The raw materials for the synthesis of the chemical constituents of proto- 

 plasm are to be found in the yolk, which is laid down in the egg during its 

 growth period in the ovary. The frog egg contains large yolk platelets com- 

 posed chiefly of protein. The platelets break down during development, and 

 the proteins are hydrolyzed and provide the raw materials for the synthesis 

 of the proteins of the new cells. It is during the process of differentiation of 

 an organ that the yolk platelets in its cells breakdown. 



The mechanism of yolk distribution to the cells of the frog embryo is 

 simple. Cell division simply cuts off some of the yolk in each of the cells of 

 the embryo. The chick egg is so large and there is so much yolk that the 

 yolk is not incorporated into the cells of the embryo and remains a non- 

 cellular mass. Thus in the development of the chick embryo the transport 

 of the raw materials from the yolk to the growing cells in the embryo forms a 

 special problem. The transport is brought about by the yolk sac and blood 

 system of the chick embryo. Let us examine this system briefly. 



During early development the blastodisk of the chick embryo takes up its 

 raw materials from the immediate surroundings. As long as the embryo is 

 small, the yolk underneath the blastodisk and at the edge of the blastodisk 



