106 CHARLES R. STOCKARD 



investigation of the development of the living yolk-sac would be 

 most instructive for a comprehensive knowledge of the behavior 

 of mesenchyme in forming the blood cells and vessels. The yolk- 

 sac had been thoroughly investigated in sections and the ap- 

 pearance and location of the earliest blood islands and vascular 

 formations were already familiar. 



The egg of the Teleost is particularly adapted to the investi- 

 gation of such a problem, since its yolk-sac has no definite mesen- 

 chymal layer and the freely wandering mesenchyme cells may be 

 clearly seen between the ectoderm and yolk periblast. The 

 remarkable extent to which the cells migrate and the great 

 numbers of such wandering cells impress one with the im- 

 portance of this cellular movement in embryonic development. 

 The appreciation of this phenomenon also emphasizes the great 

 danger of interpreting developmental processes from a mere 

 study of serial sections. Sections fail to produce a correct 

 impression of what is actually taking place in an area vasculosa. 

 The study of living embryos is absolutely necessary and through 

 it one quickly becomes acquainted with the remarkable role 

 played by wandering cells in the formation of the heart and 

 vessels, as well as the production of the future blood cells. 



These facts have been pointed out long ago, but with little 

 effect, as is indicated by the enormous literature containing the 

 endless interpretations and guesses of numerous authors after 

 studies of fixed and sectioned material. It is not intended to 

 under rate the importance of the study of sections. However, 

 such a study to be fully comprehended must be accompanied 

 by observations on the living material as far as is practical. 

 These observations are further greatly clarifed by an experi- 

 mental modification of the normal developmental processes where 

 such is possible. 



Almost thirty years ago Wenckebach ('86), at that time a 

 young medical student in Holland, described his observations on 

 the living embryos of the developing bony-fish. In this con- 

 tribution he lamented the fact that the knowledge of the embry- 

 ology of the bony-fish, as well as of other vertebrates, was based 

 almost entirely on studies of sections of the embryos. The 



