THE PHYSIOLOGY OF DEVELOPMENT 201 



be made to deal with that large body of inquiry in the field 

 of " experimental embryology " which is not as yet susceptible 

 to quantitative treatment * ; we shall merely attempt to indicate 

 directions in which what are ordinarily called physiological 

 methods have been brought to bear upon developmental 

 phenomena. As the subject is a difficult one a certain amount 

 of latitude in defining concepts which may assist to clarify 

 the issues may be permitted. 



The normal end-product of development — the individual 

 as we know it — is only one of a large number of ways in which 

 the hereditary constitution can be reahsed spatially. By the 

 methods of regeneration and implantation of organs, and by 

 varying the physicochemical constituents of the external 

 medium other structural patterns can be induced. Modifica- 

 tion by physicochemical agencies alone falls within the scope 

 of this treatment. As in deaHng with the problems of fertilisa- 

 tion, if the experimentalist can modify the course of events by 

 physicochemical means, some progress will have been made 

 towards an understanding of the mechanical basis of the 

 natural process. A measure of success has already attended 

 the efforts of experimentalists in modifying the course of 

 individuation by physicochemical agencies. One may recall 

 the well-known experiments of Stockard (1906), who found 

 that by placing eggs of the Atlantic minnow in a mixture of 

 sea water and magnesium chloride (19/60 M) about half 

 the individuals developed into one-eyed forms. These 

 Cyclopean monsters were of two varieties ; in one kind the 

 two optic rudiments approximated at an early stage in the mid- 

 dorsal line and coalesced ; in the other only one eye developed, 

 shifting into a median dorsal situation. Many of these 

 embryos hatched out, and were able to swim like the normal 

 individual. Again, there is the well-known method, due to 

 Herlitzka, of producing Siamese twins in newts by mechanical 

 means. If the two cells of the first cleavage in the newt's 

 egg are separated by a fine noose of hair in the plane of the 

 first furrow each half may segment as a whole, developing 



* On this, readers should consult Jenkinson (1909). Diirken (i9i9)> 

 and Wilson (1925). 



