Scientific Proceedings. 



141 



brought under direct observation while alive, in order that a cor- 

 rect conception might be had regarding what takes place as the 

 fiber extends during embryonic development from the nerve center 

 out to the periphery. 



The method employed was to isolate pieces of embryonic 

 tissue known to give rise to nerve fibers, as for example, the 

 whole or fragments of the medullary tube, or ectoderm from the 

 branchial region, and to observe their further development. The 

 pieces were taken from frog embryos about 3 mm. long, at 

 which stage, i. e. y shortly after the closure of the medullary folds, 

 there is no visible differentiation of the nerve elements. After 

 carefully dissecting it out the piece of tissue is removed by a fine 

 pipette to a cover slip upon which is a drop of lymph freshly drawn 

 from one of the lymph sacs of an adult frog. The lymph clots 

 very quickly, holding the tissue in a fixed position. The cover 

 slip is then inverted over a hollow slide and the rim sealed with 

 paraffine. When reasonable aseptic precautions are taken, tissues 

 will live under these conditions for a week and in some cases 

 specimens have been kept alive for nearly four weeks. Such 

 specimens may be readily observed from day to day under highly 

 magnifying powers. 



While the cell aggregates, which make up the different organs 

 and organ complexes of the embryo, do not undergo normal trans- 

 formation in form, owing no doubt in part to the abnormal condi- 

 tions of mechanical tension to which they are subjected, neverthe- 

 less the individual tissue elements do differentiate characteristically. 

 Groups of epidermis cells round themselves off into little spheres 

 or stretch out into long bands, their cilia remain active for a week 

 or more and a typical cuticular border develops. Masses of cells 

 taken from the myotomes differentiate into muscle fibers showing 

 fibrillae with typical striations. When portions of myotomes are 

 left attached to a piece of the medullary cord the muscle fibers 

 which develop will, after two or three days, exhibit frequent con- 

 tractions. In pieces of nervous tissue numerous fibers are formed, 

 though owing to the fact that they are developed largely within 

 the mass of transplanted tissue itself, their mode of development 

 cannot always be followed. However, in a large number of cases 

 fibers were observed which left the mass of nerve tissue and ex- 



