REGENERATION 517 



OBSEBVATIONS AMD BECORDING 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. Such a record should consist of 

 drawings or photographs taken at stated Intervals (not more than 2k hours, in most cases) 

 ■beginning Immediately after the operation. The dates for all drawings, the temperature of 

 the medium, the conditions of food, light, space, etc., must all he recorded In the vari- 

 ous 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 inrportant role. The exact 

 method of this repair is not thoroughly understood. It is not estahlished that such re- 

 pair is the same for all animals, or at all stages within the life span of a single organ- 

 ism. 



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

 some who hell eve that there are reserve, mesenchyme-type cells in all organs, awaiting 

 call for the specific function of regeneration. Ther« is no doubt that an injury calls 

 for the marshalling of active cells in the vicinity of the cut, hut many of these calls 

 are of vascular xsrigin 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-differen- 

 tiation, to be followed by an indifferent (embryonic) period, and then a re-differentia- 

 tion, either into similar or dissimilar tissues. Some rigid adherents to this concept be- 

 lieve that re-differentiation 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 ( 19^+0) says: "Perhaps the more widely held view, is that organ- 

 ismic control is established and maintained by reason of the diffusion of specific organ- 

 izing 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 excercises which deal specifical- 

 ly with certain organ systems such as THE EYE, THE HEART, THE LIMB FIELDS, etc. However, 

 this exercise will illustrate the principles Involved and also the fact that Amphibian 

 larvae do exhibit remarkable powers of regeneration. 



Emerson {19'+1) has shown that parts of the early and late gastrula ectoderm can be 

 Implanted into a tail blastema and will differentiate into recognizable organs. This is 

 therefore another method (in addition to isolation culturing) of determining the prospec- 

 tive potencies of various fields or areas of the early gastrula or, in fact, any early 

 embryonic stage. Grafts into the larval tall blastema may survive as long as 100 days 

 after transplantation (Emerson, 19'+'+), 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 tall. Differentiation is achieved within 

 a week or two, and many of the grafts will disintegrate at this time. In most cases re- 

 sorption of the graft is accomplished along with the resorption of the tall at the time 

 of metamorphosis. Sometimes, however, a part of the graft may persist and will be found 

 at the tip of the urostyle of the metamorphosed frog. 



The tadpole tail regenerates rapidly. This may be due in part to the embryonic 

 nature of the cells contiguous to the cut surface. The initial axis of regeneration is 

 generally at right angles to the cut surface but this is rectified subsequently. Twitty 

 and Delanney (1959) found that the tail of Amblystoma is capable of repeated regenerations 

 after successive amputations even during long periods of continuous starvation. There is 

 a gradual decrease in the rate (and degree) of regeneration as amputations are performed 

 later in larval life (Goodwin, 19'+6) and newly hatched larvae respond the better because 

 they have incompletely differentiated tissue which can be more simply de-differentiated 

 to form a blastema, and then re-dif ferentlate as required. 



