676 



Regeneration 



of the histological processes. The new rays 

 are formed by osteoblasts which form at 

 the tip of the cut rays, and the horny rays, 

 which in development are simultaneovis with 

 the dermal rays, are formed later. 



While the experiments have been per- 

 formed usually on the tail fins or dorsal 

 fins, the anal and paired fins give the same 

 result. Scott ('07) showed that in young 

 fish the capacity for regeneration is greater 

 and the time during which it is completed 



faster and complete the process much more 

 quickly both actually and relatively. Sum- 

 ner ('04) had shown that the embryo would 

 repair parts of the neural plate after injury, 

 as would also the shield. Hoadley ('28) later 

 repeated this experiment and likewise re- 

 moved blastomeres with no defect produced 

 in the later embryo. Lewis ('12) found that 

 midline injuries in the plate tended to leave 

 definite defects in the resulting nervous 

 system, which finding Hoadley later corrob- 



Fig. 233. 



is less than in older fish. Nusbaum and 

 Sidoriak ('00) studied trout fry, in which 

 regeneration of the entire posterior end of 

 the body will occur if cuts are made anterior 

 to the anus. This process is complete and 

 regulatory in detail even to the openings 

 of the ducts into the cloaca. Duncker ('05, 

 '07) showed in lophobranchs that skeleton, 

 muscle, fin and dermal rays can be regener- 

 ated in remarkably constant form. Morrill 

 ('06) worked on the operculum and reported 

 negatively. Beigel ('10) found that regener- 

 ation was complete but extended over a very 

 long period. 



The regeneration of skin and scales was 

 described in the papers of Fraisse (1885) 

 and Beigel ('12). The central nervous system 

 was most thorovighly treated in Hooker's 

 ('30, '32) experiments performed on Lebistes, 

 in which the spinal cord regenerates with as 

 great or greater completeness than is found 

 in frog tadpoles (Hooker, '25) and in much 

 the same manner. If young fry regenerated 

 fast, younger animals should regenerate 



orated. Oppenheimer ('36) and Sumner ('04) 

 both got perfect embryos, Sumner after in- 

 juries to the shield in the region of the 

 blastopore, while Oppenheimer removed all 

 of the gastrular dorsal lip and still got a 

 normal embryo. Some of this information 

 has been added since the original experi- 

 ments were planned, and the exceedingly 

 critical and precise work of Oppenheimer 

 ('36) has given a practical cell lineage 

 which shows what some of these cells nor- 

 mally do in this makeup of the embryo. 



Nicholas ('27) and Nicholas and Oppen- 

 heimer ('42) removed the ear, eye, fin, and 

 nerve cord at various levels, and finally the 

 tail. These operations were performed on 

 non-motile forms at the 12-somite stage. 

 None of the structures regenerated. At this 

 particular stage and for a short while there- 

 after, there is no demonstrable regenerative 

 potency; the organism is mosaic and any 

 part removed is missing, and no initiation 

 of the reconstitutive process occurs (Fig. 233). 



After the later differentiation of definite 



