III. 9 SUMMARY 157 



normal, specific growth and differentiation, and for the solution 

 of this problem only those experiments, of course, are of avail in 

 which such factors are either altered or removed. 



By this means, as we have seen, it has been shown that 

 a certain constitution of the physical environment, fixed within 

 certain limits, is needful for the embryo ; to these conditions it is 

 closely adapted ; those limits it can only transgress under pain 

 of abnormality or death. 



Every factor, or nearly every factor, is necessary for this or 

 that phase or part of the process, some for the whole. Light 

 of a certain wave-length will accelerate development, light of 

 another kind, or in some instances darkness, will retard it, or stop 

 it altogether ; a certain degree of heat is indispensable ; oxygen 

 is required for respiration, water for growth ; some eggs demand 

 constant agitation, others comparative rest; fertilization, or 

 segmentation, or gastrulation, or some one or other of the later 

 phases of development may depend absolutely on the presence 

 of some particular chemical element; remove the factor in 

 question, whatever it may be, and that particular process will 

 not occur, and the specific, typical end which is reached in 

 normal development will not be attained. Nevertheless, the 

 achievement of this end does not depend wholly upon extrinsic 

 forces, for the ovum is no completely homogeneous ' isotropic ' 

 substance in which the complex circumstances of its environ- 

 ment conspire to produce heterogeneity and coherence. There is 

 no evidence that any physical factor exerts a directive influence 

 sufficient of itself to determine any part of the whole specific 

 effect, although this may happen under extraordinary conditions, 

 as when gravity impresses a bilateral symmetry upon the com- 

 pulsorily upturned egg of the Frog, and so determines the 

 median plane of the embryo. 



Intimately bound up though these external conditions are 

 with the proper conduct of the whole series of events whereby 

 the organism comes gradually to resemble the parents that gave 

 it birth, they can only operate in conjunction with internal 

 factors which must be sought for not only in the initial structure 

 and constitution of the germ -cells, but in the mutual interactions 

 of the developing parts. 



