454 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



seem to indicate. The parts are not necessarily present in these stages, 

 but the future pattern is projected on a present pattern, from which it 

 originates. The cell migrations constitute the basis for projection. That 

 dynamic factors of some sort play a part in determining and directing 

 these movements seems probable, and the possibility that electric-poten- 

 tial gradients may be concerned in determining the differential in the 

 cells suggests itself. An anteroposterior potential gradient is reported in 

 Ambly stoma and chick embryos, with increase in potential difference with 

 progress of development and demonstrable without direct contact with 

 the embryo. Also, the dorsal lip region of Amhlystoma is electronegative 



Fig. iss, .l-C— Cell migrations in chick embryo, according to Wetzel; change in position 

 of embryonic regions indicated by gradations of stippling. A, directions in which migrations 

 will occur in formation of primitive streak; B, C, later streak stages (from Wetzel, 1929). 



to the apical pole. The developing embryo apparently gives rise to a 

 steady-state electrodynamic field with definite axiate and regional inten- 

 sities (Burr and Hovland, 1937^, h). 



The recent studies of cell migrations by means of local staining have 

 finally shown that the much debated concrescence theory of vertebrate 

 development, that all except the most anterior parts are formed by union 

 of two originally separated lateral halves, is not entirely in accord with 

 the facts. The axial organs are primarily median, but there is concres- 

 cence or convergence of certain lateral areas toward the median plane.'' 



NEURAL INDUCTION IN AMPHIBIAN DEVELOPMENT 



The work on induction and so-called "organizers" in amphibian de- 

 velopment has attracted much attention and has become widely known 



7 Sec the discussion by Vogt, 1929, pp. 668-78. 



