REGENERATION 309 



d. Add more balsam, if necessary, and cover A'ith coverglass over adequate 

 glass chip supports to prevent compression. Allow to harden completely 

 in a flat position. The cartilage will be blue and white distinct from the 

 other tissues. 



OBSERVATIONS AND RECORDING OF DATA: 



There are three distinct parts to this exercise on Regeneration and for each specimen 

 studied there must be a complete and separate record or case history. Such a record 

 should consist of drawings or photographs taken at stated intervals (not more than 24 

 hours, in most cases) beginning immediately after the operation. The dates for all draw- 

 ings, the temperature of the medium, the conditions of food, light, space, etc. , must all 

 be recorded in the various places provided. 



DISCUSSION: 



Self-repair is a characteristic of all protoplasm, a necessary prerequisite in a com- 

 petitive environment where natural selection plays such an iinportant role. The exact 

 method of this repair is not thoroughly understood. It is not established that such repair 

 is the same for all animals, or at all stages within the life span of a single organism. 



There are still two main concepts relative to the method of restitution. There are some 

 who believe that there are reserve, mesenchyme-type cells in all organs, awaiting call 

 for the specific function of regeneration. There is no doubt that an injury calls for the 

 marshalling of active cells in the vicinity of the cut, but many of these calls are of vas- 

 cular origin and may have nothing to do with regeneration. The second concept is that the 

 injured cells at the cut surface, and nearby, undergo a period of de-differentiation, to be 

 followed by an indifferent (embryonic) period, and then a re-differentiation, either into 

 similar or dissimilar tissues. Some rigid adherents to this concept believe that re-dif- 

 ferentiation can only be along the original lines of differentiation, implying incomplete 

 de -differentiation. There is, of course, controverting evidence against this. In general 

 the regenerated part does resemble in structure and in function the lost part. Buchanan 

 (1940) says: "Perhaps the more widely held view, is that organisnnic control is estab- 

 lished and maintained by reason of the diffusion of specific organizing substances arising 

 as the result of specific metabolisms of organizing or inducing centers." 



Regeneration is not limited to the structures or anlagen listed in this exercise, and minor 

 experiments in regeneration are listed in other exercises which deal specifically with cer- 

 tain organ systems such as THE EYE, THE HEART, THE LIMB FIELDS, etc. How- 

 ever, this exercise will illustrate the principles involved and also the fact that Amphibian 

 larvae do exhibit remarkable powers of regeneration. 



Emerson (1941) has shown that parts of the early and late gastruia ectoderm can be im- 

 planted into a tail blastema and will differentiate into recognizable organs. This is there- 

 fore another method (in addition to isolation culturing) of determining the prospective 

 potencies of various fields or areas of the early gastruia or, in fact, any early embryonic 

 stage. Grafts into the larval tail blastema may survive as long as 100 days after trans- 

 plantation (Emerson, 1944), and the differentiations may include eyes with lenses, brain 

 parts, striated muscle and cartilage. This period of 100 days is long after the blastema 

 has become an integral part of the tail. Differentiation is achieved with a week or two, 

 and many of the grafts will disintegrate at this time. In most cases resorption of the 

 graft is accomplished along with the resorption of the tail at the time of metamorphosis. 

 Sometinnes, however, a part of the graft may persist and will be found at the tip of the 

 urostyle of the metamorphosed frog. 



