66 The Development of the Vertebrate Embryo 



segmented into blocks of tissue called somites. The somite cells migrate 

 toward the notochord and the spinal cord and form the vertebrae (as 

 described previously) and also skeletal muscles. They also spread upward 

 to underlie the outer ectoderm and become part of the skin and inward 

 to become the outer covering of internal organs. Portions of the mesoderm 

 lateral to the somites pinch off as longitudinal strips of tissue and give rise 

 to the urogenital tract, including kidneys, gonads, etc. The heart and 

 circulatory system also originate from the non-somite mesoderm. 



DEVELPOMENT OF LIMBS 



The limbs first appear as slight external swellings ( limb buds ) which 

 rapidly elongate. The bud is simply a mass of loose mesodermal tissue 

 capped by an ectodermal cover. As the limb elongates, the mesodermal 

 cells grow rapidly in number and come together into a series of tight ag- 

 gregates corresponding to the skeletal elements of the adult hmb. They 

 then are transformed into cartilage and finally into bone tissue. The re- 

 maining mesoderm forms muscles and blood vessels. Meanwhile, the 

 external sculpturing of the limb, joints, digits, etc. is accomplished. 



Biochemical Embryology 



We are accustomed to the fact that the specialized cells of a multi- 

 cellular organism have vastly diflFerent structures and functions. I have 

 mentioned before, but cannot stress too greatly, the fact that these are 

 simply outward manifestations of differences in metabolism, in the spectra 

 of chemical reactions which in turn reflect what enzymes and enzyme 

 concentrations these cells possess. Therefore, merely to describe the ac- 

 quisition of new structure by a developing embryo would be incomplete 

 without a concomitant catalog of biochemical changes. 



As a result of biochemical research over the past thirty years, a mul- 

 titude of enzymes has been discovered, purified, and characterized. The 

 reactions that they catalyze have been studied in the test tube to the point 

 where we can specify the reacting molecules, the products, and the chem- 

 ical mechanisms by which reagents are transformed into products. These 

 reactions, each representing a relatively minor chemical alteration, have 

 been integrated into comprehensive reaction chains and cycles that ex- 

 plain the broad pathways of breakdown and synthesis of cell components 

 and of energy generation, transport, and utilization. This in turn has led 

 to a rational basis for understanding the nutritional requirements of or- 

 ganisms (i.e., the starting materials needed for synthesis and energy gen- 

 eration). In addition, the chemistry of the major cell components, pro- 



