270 PATTERNS AND PROBLEMS OF DEVELOPMENT 



mental teratogeny lead us to believe that the determining factors may 

 differ widely even though the modifications are similar. 



Apparently inhibition in earlier stages may produce cyclopia in verte- 

 brates; in later stages after normal localization of optic primordia, only 

 microphthalmia. The occurrence of cyclopia has suggested that the optic 

 primordium may be primarily median and become bilateral later and that 

 the inhibiting conditions merely prevent the secondary change in locali- 

 zation. This question is discussed later in connection with certain trans- 

 plantation experiments which bear upon it (pp. 282-85). 



CONCLUSION 



Differential modifications of development indicate presence of physio- 

 logical factors of developmental pattern which are very similar as regards 

 regional differences and relations in various animal groups, from coelen- 

 terates and flatworms to the higher vertebrates. The fact that a large 

 number of external agents, both chemical and physical, within certain 

 ranges of concentration or intensity, produce similar modifications of early 

 development with axial gradations in degree of modification indicates 

 that the physiological factors on which the modifications depend consti- 

 tute a quantitative gradient pattern, rather than a regionally specific 

 pattern of different substances and metabolisms. The possibihty that a 

 regionally specific pattern may also be present is, of course, not excluded 

 by the characteristics of the modifications; but if it is present, it is evi- 

 dently not the chief factor in determining the similar graded characteris- 

 tics of the differential modifications. Moreover, the alterations in locali- 

 zation of particular differentiations and the complete obliteration of axiate 

 pattern and all localized differentiation by differential inhibition and ob- 

 Hteration of the gradient pattern suggests that differentiations are de- 

 pendent on, and results of, this pattern. 



The various lines of evidence considered in this and preceding chap- 

 ters indicate that axiate pattern is primarily a quantitative gradient pat- 

 tern, involving the essential activities of living. Within this general pat- 

 tern specific differences of substance and reaction arise and become the 

 basis of differentiations. With the changes in activity in the course of 

 development new centers of activity and new gradient systems may arise, 

 some of them with metabolisms different in character from that of the 

 primary pattern. As these differences increase, they affect activities in 

 other parts, and physiological interrelations and integrating factors be- 

 come increasingly effective and varied in character. Apparently, how- 

 ever, quantitative gradient factors may persist and be physiologically ef- 



