294 Cell as Basis of Organic Activity 



the other hand, have indeterminate cleavage. If blastomeres are isolated 

 early enough, each will develop into a new individual. 



Transplants of Embryonic Tissues. — The blastomeres of most 

 vertebrates potentially are capable of developing into nearly any type 

 of tissue. They can be moved from one position to another in an embryo 

 without disturbing the normal development. After gastrulation, how- 

 ever, this generali23ed ability of the embryonic tissue is lost, and the 

 individual cells are capable of developing only into certain specific 

 structures. Much of this information has been gathered from work with 

 the amphibians, but apparently the knowledge so gained can be applied 

 to other forms. Thus it can be shown that if an eye bud is transplanted 

 to the leg region, it will still develop into an eye. With these experi- 

 ments many quite unusual salamanders have been produced ! Some have 

 extra legs on their backs, eyes on their tails, etc. The results of these 

 experiments have given us a clearer picture of the importance of tem- 

 poral events in the differentiation of the embryo. 



These transplantation experiments denitely show that there is a 

 point at which the individual cells lose their extensive potentialities 

 and become limited to a definite pattern of action and structural differ- 

 entiation. In those animals having determinate cleavage, this pattern 

 is fixed in the eg^; in others, this pattern is apparently not fixed until 

 about gastrulation. 



Embryonic Determiners or Organizers. — When the cell loses 

 its extensive potentialities apparently some chemical change occurs 

 within it. This change may occur within the cell itself, or very commonly 

 a whole group of cells is affected. Certain areas of the embryo are able 

 to control the fate of surrounding cell masses. These parts, known as 

 organisers, are the source of chemical substances which are transmitted 

 from one part of the embryo to another. 



One of the earliest regions to assume the role of organizer is the 

 dorsal lip of the blastopore. This, it will be recalled, is the area where 

 part of the mesoderm and the notchord arise. This area induces the 

 formation of the neural plate. If transplanted to another embryo, it 

 will induce similar formation in this foreign ectoderm. 



'b' 



In those embryos having meroblastic cleavage, a similar organizer 

 is noted in the region of the primitive streak. It also induces the 

 formation of the neural plate. 



