GROWTH, REGENERATION AND REPRODUCTION 1115 



extreme tardiness. Harrison, 1 however, has proved that nerve cells 

 may also be grown outside the body in suitable culture media. When 

 clotted lymph is used, the cell-body grows and sends out its oxone and 

 dendrites which may be traced far into the surrounding medium. 

 This observation also proves that nerve fibers are the outgrowths of 

 the hyaline protoplasm of the nerve cells which at this stage of develop- 

 ment is actively ameboid. These long drawn out pseudopodia 

 eventually become the organized fiber processes. Consequently, 

 the central complexes of ganglion cells must exert a commanding 

 influence upon the development of the fiber paths. This view finds 

 substantiation in the fact that the transplanted limbs of the embryos 

 of the toad and frog eventually acquire a normally arranged system of 

 nerves. 2 No matter where the new limb is united with the body, 

 these nerves show a perfectly normal distribution in relation to those 

 of the host. Thus, a limb implanted in the region of the head, in- 

 variably acquires nervous outgrowths which are derived in regular 

 order from the facial nerve or some other nerve, if closer to the graft. 

 In this category also belong the morphological and embryological 

 experiments of Pfliiger, Roux, Born, and others, purposing to test the 

 regenerative powers of animals when injured during their period of 

 development or when the organic constituents of the egg itself are 

 either removed or transplanted from one animal to another. One 

 of the most interesting discoveries was made by Born 3 in 1894. While 

 performing certain experiments pertaining to the reformation of lost 

 parts of the embryo of the frog, he found that pieces which had first 

 been absolutely separated from the main mass, might again be made to 

 unite with it by simply holding them against it for a few hours. This 

 preliminary fact having been established, he then succeeded in uniting 

 these pieces in all possible ways, producing even monsters with two 

 tails or two heads or a head in the place where the tail ought to be. 

 Even pieces from different animals could be used in the production 

 of these odd forms. With the help of the Zeiss binocular dissecting 

 microscope and delicate instruments, Spemann 4 was able to perform 

 transplantations of much greater delicacy than those just related. 

 These included the removal of certain areas of the epidermis or of the 

 Gasserian ganglion and their implantation in some other part of the 

 body; the removal and reversal of the auditory vesicle, as well as the 

 interchange of the right and left ears. By the same means Lewis 5 

 proved later on that the epidermis of any part of the body may be 

 brought into contact with the optic vesicle at the proper stage of 

 development and give rise to a crystalline lens. 



1 Proc. Soc. for Esp. Biolog. and Med., 1907. 



2 Held, Verhandl. der anat. Gesellsch., Rostock, 1906, and Harrison, Jour, 

 of Exp. Zoology, iv, 1907. 



3 Archiv fiir Entwickelungsmechanik, iv, 1896-1897; also Brans, Propfung bei 

 Tieren, Verhandl. des naturhist. med. Vereins, Heidelberg, iii. 



4 Verhandl. der deutsch. zoolog. Gesellsch., 1906. 



5 Am. Jour, of Anat., iii, 1904, and Jour, of Exp. Zoolog., ii, 1905. 



