746 LIFE : OUTLINES OF GENERAL BIOLOGY 



and (c) the environmental influences (pressure, osmosis, chemical 

 composition of the medium, aeration, temperature, light, and so 

 forth) which play upon the developing whole. By ingenious methods 

 the experimental embryologists have of recent years greatly advanced 

 our understanding of these factors. 



As a method in investigating both the structural and the func- 

 tional aspects of development, experiments have proved very 

 illuminating, and they have been of many kinds, utilised at many 

 different levels. It is convenient to refer to these under the title 

 Experimental Embryology; though it is plain enough that this does 

 not form a subdivision of Embryology. There is anatomical embryo- 

 graphy and physiological embry ography ; but "experimental em- 

 bryology" is simply a summation of the results reached by using 

 experimental methods. It is thus not a distinct department, but 

 simply the application of various technical methods, as of modifying 

 environment, etc., etc., so often giving rise to changes of function, 

 and even form, by which fresh light is often thrown upon the normal 

 physiology and anatomy of the embryo. 



Here the fullest order of treatment would begin with the earliest 

 stages of development, and work onwards; showing how experiments 

 have been made with the maturation of the germ-cells, their fertilisa- 

 tion, their segmentation, the establishment of germinal layers, the 

 process of organogenesis, and so onwards in development. Yet, as 

 more within the limits of this introductory outline, let us give 

 examples of different kinds of experiments, and arrange these as far 

 as possible in the order of their application. This has the advantage 

 of gradually leading up to the most modern experiments, notably 

 those of Spemann, which are certainly the subtlest and most 

 suggestive. 



(i) Monstrosities . — The idea of interfering with the embryo is an 

 old one. Thus Swammerdam in the seventeenth century is said to 

 have succeeded in producing monstrosities ; but for practical purposes 

 this type of experimental embryology dates from the last quarter of 

 the nineteenth century, with Dareste (Recherches sur la production 

 artificielle des Monstruo sites; ou Essais de Teratogenie Experimentale, 

 Paris, 1877). He experimented chiefly with the developing egg of the 

 fowl, placing it vertically instead of horizontally, keeping it slightly 

 above or slightly below the normal temperature of incubation, 

 heating different parts unequally, hermetically varnishing part of 

 the shell, and so on. His suggestive work was justified in two ways, 

 first in the production of sundry malformations which throw light 

 on normal development, and second in showing that the developing 

 embryo is plastic, yielding to changes in its environment. Develop- 

 ment is thus a resultant of hereditary nature and environing 

 "nurture" in the widest sense. 



The next pioneering inquiry was A. Rauber's Formbildung und 



