SECT. 3] AND ORGANISATION 601 



gradient in it. No further changes occur during elongation and 

 hatching. 



The gradients in the chick embryo were also studied by Hyman, 

 using potassium cyanide, and ammonium and sodium hydroxides, 

 sometimes preceded by staining with neutral red. The earliest stages 

 were very difficult to deal with, but some evidence was obtained of 

 an antero-posterior gradient in the central opaque area of the 

 germinal disc at 7 hours' incubation. This was quite certainly 

 demonstrable, however, at the typical primitive streak stage, and 

 the stage of the head process. The medullary plate stage marked 

 the beginning of the double gradient (see Fig. 102), two regions 

 of high susceptibility being present, one at the anterior end of the 

 primitive streak and the other at the anterior end of the medullary 

 plate. When the first somites appear the same zones are seen, the 

 former spreading backwards and forwards along the embryonic axis, 

 the latter backwards only. As the neural folds close, they present 

 a region of high susceptibiUty, but this soon disappears, and the 

 embryo reverts to the simple double antero-posterior gradient system. 

 This holds good up to the 8-somite stage; from the 9th onwards the 

 rapidly increasing susceptibility of the optic cups is noticeable. At 

 the i2-somite stage the optic zone has died away and there is a 

 new one of high susceptibility in the hind brain, foreshadowing 

 the turning of the head, but this also disappears by the 3rd day 

 of development. Summing up the results, one may say that the 

 general picture is one of an antero-posterior gradient complicated 

 from time to time by the appearance of zones of high susceptibility 

 at different points along the embryonic axis. 



In vertebrate embryos in general, it would seem that the forma- 

 tion of these two regions of high susceptibility is the regular mode 

 of development. The two centres are always located in the same 

 position with respect to the future embryo, one at the anterior end 

 of the antero-posterior axis and one in the axis at a more or less 

 posterior point. This posterior centre is the dorsal lip of the blastopore 

 in cyclostome and amphibian embryos, the posterior end of the 

 embryonic axis in teleostean fishes, and the primitive knot, sub- 

 sequently the tail bud, in the chick. "This posterior centre", says 

 Hyman, "is Hke a growing point which, passing backwards, deposits 

 the trunk of the embryo anterior to it." The presence of two centres 

 of activity was long ago recognised in frog and rabbit embryos 



