538 THE DEVELOPMENT OF THE INDIVIDUAL 



Experimental Embryology 



In recent years there has been some very important work performed 

 on early embryos by means of delicate microdissection instruments. 

 Small bits of tissue have been transplanted from one part of the embryo 

 to other parts of the same or different embryos. The growth and dif- 

 ferentiation of these transplants have gone a long way toward the ex- 

 planation of cellular development. 



In a frog or salamander embryo, if a portion of the dorsal lip of the 

 blastopore is excised from one embryo and transplanted to the ventral 

 region of another embryo, two sets of organs will be formed. The 

 transplanted blastopore tissue acts as an organizer and forms another 

 neural tube, notochord, a double set of somites, and an archenteron at 

 the same time as the normal blastopore of the embryo forms the regular 

 set. 



This power of induction by the cells of the dorsal lip region is due to 

 chemical substances that diffuse into the surrounding cells. This tissue 

 may be killed by boiling, freezing, or by chemicals without destroying its 

 power to organize a set of tissues when grafted to a young embryo in 

 the early gastrula stage. 



In the very early gastrula when the dorsal lip of the blastopore has 

 just begun to form, experiments have shown that the various germ 

 layers are still undifferentiated. If a piece of tissue from the region of 

 the future spinal cord is grafted to the region of future skin, the piece 

 develops into ordinary skin and the skin grafted to the spinal cord 

 region develops into the spinal cord. A little later, however, when the 

 embryo has come under the influence of the organizer center during 

 gastrulation, the same experiment yields exactly opposite results. The 

 spinal cord tissue grafted to the belly region develops a tiny piece of 

 spinal cord, and the skin grafted into the spinal cord region develops a 

 piece of skin in the middle of the spinal cord. Even though all the cells 

 still contain a full supply of genes, changes have taken place in the 

 cytoplasm which restrict these cells to the formation of certain organs. 

 Hans Spemann, the German embryologist who made these important 

 studies, was awarded the Nobel Prize in medicine for explaining the 

 mechanism of embryological specialization which had mystified biolo- 

 gists for centuries. Spemann further showed that the original organizer 

 actually started a chain of organizers which continued the differentia- 

 tion of organ after organ. For example, the brain forms the optic 

 vesicle which in turn stimulates the adjacent ectoderm to form a lens. 

 Such an optic vesicle, when inserted under the skin on the side of the 

 body, will organize the overlying ectoderm into a lens. 



