The Development of the Vertebrate Embryo 67 



teins, nucleic acids, polysaccharides, etc., has been elucidated, and the 

 relationships between the structure of cell organelles and their biochemi- 

 cal functions have begun to emerge. 



Until recently, however, most biochemical studies have been done 

 with bacteria harvested after they had stopped growing or with adult 

 animal tissues and cells ( liver or muscle slices, etc. ) . Consequently, they 

 could tell us only about the biochemical machinery of a cell at one instant 

 in time, so to speak. But with our present biochemical understanding, it is 

 now possible to examine how the biochemical apparatus changes as the 

 organism proceeds upon its developmental course. At this early stage, 

 some of the directions taken by students of biochemical embryology are 

 outlined below: 



1. Chemical characterization of the egg. For example, we can ask: 

 do the components of an egg differ in kind or quantity from those of an 

 ordinary cell? Do the organelles (nucleus, mitochondria, ergastoplasm, 

 etc.) have the same chemical composition and biochemical activities as 

 they do in ordinary cells? What are the components of yolk? How are they 

 utilized? 



2. Biochemistry of spermatozoa. What are the energy requirements 

 for sperm locomotion? What substrate is used ( the sugar, fructose ) ? How 

 (via the glycolytic cycle, as explained in the monograph on cell physi- 

 ology, and therefore much like other cells ) ? A very interesting question is 

 how the sperm midpiece synthesizes the protein fibers that make up the 

 spindle apparatus for the first cleavage. 



3. Cleavage. A great deal of work has already been done, partic- 

 ularly with the sea-urchin blastula. As mentioned previously, the egg starts 

 out with very disproportionate amounts of nuclear and cytoplasmic ma- 

 terials. As cleavage proceeds, the ratio of the two rises to a normal bal- 

 ance. We want to know about the chemical details of the shift and of the 

 controlling mechanisms. We want to know why the first cleavages are 

 synchronous and why later on some parts of the blastula cleave faster 

 than others. 



4. Nutrition of the embryo. For example, early chick embryos have 

 been separated from their yolk supply and placed on agar medium con- 

 taining amino acids, vitamins, mineral salts, and glucose as an energy 

 source. The embryos developed in normal fashion under these conditions 

 and formed a full complement of organs. We can ask: what are the mini- 

 mum nutritional requirements for normal embryonic development? What 

 are the biochemical pathways of energy generation and how is the energy 

 employed for cellular differentiation, morphogenesis, and growth? If we 

 omit a specific essential nutrient, does all embryogenesis cease or are only 

 certain organs affected? 



