107 



ease with which the limb regenerates , in these cases is no doubt 

 dependent on the undifferentiated condition of the tissues generally 

 at this period. Never-the-less, it is an interesting fact that an entire 

 new limb should be able to form in these embryos out of tissue that 

 was originally in a more ventral position in the body wall, and was 

 brought secondarily into the limb-region during the process of healing. 

 Normally this tissue must have taken part in the formation of some 

 other structures than those of the limb. 



How soon after the operation the young embryos begin to regene- 

 rate their extirpated parts and at what stages the ability to per- 

 fectly restore a lost member becomes limited, as seems to be the 

 case in older embryos, I am, at present, unable to say. I hope, 

 however, to solve some of these questions through renewed experiments. 



The other mesoblastic structures in the limb-region that were 

 injured when the limb-rudiment was extirpated, show no permanent 

 effects of the operation. Cross sections of embryos which had been 

 injured but which later regenerated their lost organs, sometimes show 

 the mesoderm of the somatopleure slightly thinner in the limb-region 

 than elsewhere. This is, however, by no means an invariable con- 

 sequence and it is often extremely difficult to tell where the injury 

 has been. Most of the embryos that were kept under favorable 

 conditions of temperature and with an abundant supply of food for a 

 few weeks, give no evidence of ever having been injured. 



These experiments have an interesting bearing on- the origin of 

 tissues in the limbs, although they cannot, alone, be considered as 

 giving conclusive proof of the somatopleuric origin of limb-muscles. 

 Never-the-less , they add important data to the evidence already 

 deduced from previous experiments 1 ) which show that the entire 

 limb is somatopleuric in origin. 



The regeneration of limbs in these embryos also furnishes some 

 ground for believing that the function of the cells in the limb-region 

 is determined by their position in the organism and by their relation 

 to other cells, rather than by any thing inherent in the cells themselves. 



In conclusion, not only do the limbs arise from the somatopleure, 

 but it would appear that they may arise from any part of the somato- 

 pleure that is brought into the limb-region, provided the transfer is 

 accomplished before differentiation has begun. 



Marine Biological Laboratory, Woods Holl, 

 Mass. U. S. A., July 16, 1898. 



1) E. F. Byknes, Journal of Morphology, Vol. 14, No. 2, 18!>8. 



