380 PROGRESS OF SCIENCE IN THE CENTURY. 



not here discuss the various theories. It may be said, 

 however, that each step in development is a function 

 of three factors: (a) the organisation of the germ- 

 cells, objectively expressed in a visible complexity of 

 structure, and in an inconceivable molecular com- 

 plexity beneath this: (&) the vital relation of the 

 various blastomeres or segmentation-cells to one an- 

 other; and (c) the environmental influences (pres- 

 sure, osmosis, chemical composition of the medium, 

 temperature, light, etc.) which play upon the whole. 



EXPERIMENTAL EMBRYOLOGY. 



Although the idea of artificially influencing the 

 germ is very old, although even Swammerdam is said 

 to have succeeded in producing monstrosities, ex- 

 perimental embryology is practically a new depar- 

 ture in biology. Almost all the experiments of mo- 

 ment have been made in the last twenty years, and 

 since 1890 it has been a prominent line of research. 

 There is a Journal Archiv fur Eniwickelungs- 

 Mechanik, edited by Roux which is in great part 

 devoted to the subject, and there are already at least 

 two text-books mainly devoted to its exposition.* 



(a) One of the first modes of experiment in this 

 direction was in the artificial production of monstros- 

 ities. Just as pathology sheds light on physiology 

 in the case of the thyroid gland for instance so 

 teratology and teratogenesis (the study and produc- 

 tion of monstrosities) may help us to understand 

 normal development. The most successful worker 

 along this line has been Camille Dareste,f the 



* W. Haacke, EntwicTcelungs-MecJianik. C. Labbe, Cytol- 

 ogie Exptrimentale. 



t Recherches sur la production artificielle des monstruos- 

 iUs; ou Essais de Ttratog&nie Expcrimentdle, Paris, 1877; 

 2nd ed., 1891. 



