378 THE FURTHER DIFFERENTIATION OF THE 



differentiating as regards certain quantitative features.^ But before 

 dealing with these factors, attention must be paid to the conditions 

 under which tracts of axons are formed and the direction of their 

 growth controlled, for they play an all-important part in the pro- 

 blem (fig. 182). 



From the pioneer experiments on tissue-culture,^ and those of 

 grafting limb-buds ('' aneurogenic ") from embryos whose spinal 

 cords had previously been extirpated,^ it is known that axons grow 

 out as free projections from the cell-bodies of the neurons. It 

 is also important to notice that experiments in which neurons 

 have been made to produce axons in tissue-cultures through 

 which an electric current is passing, show that the direction of out- 

 growth of the axon is controlled by the direction of the current. 

 Further, if a conductor carrying an electric current is passed 

 through the culture, the axons grow out from their cell-bodies in a 

 direction at right angles to the axis of the conductor.^ The strengths 

 of current used (about 2 billionths of an amp.) correspond in range 

 with those found in living embryos. 



Now, experiments on the differential susceptibility of the parts 

 of young embryos of Amhlystoma at the early tail-bud stage show 

 that two axial gradients are present. One of these appears in the 

 ectodermal tissue of the dorsal side of the body and has its high 

 end at the head, decreasing posteriorly. This gradient is clearly a 

 derivative of the original gradient of the primary egg-axis of 

 polarity. The other gradient is situated in the tissues of the noto- 

 chord and mesoderm underlying the neural tube, and has its high 

 end at the high end of the embryo, decreasing anteriorly. This has 

 been proved by susceptibility experiments.^ Since the hind end of 

 the embryo, where this second gradient has its high end, corre- 

 sponds to the point of closure of the blastopore, this gradient must 

 represent that of which the organiser was the top during earlier 

 stages of development (fig. 183). 



In the embryo of Amhlystoma at the early tail-bud stage there are 

 therefore two gradients, working in opposite directions. Now the 

 ventral part of the neural tube is in intimate contact with, and is 



^ Yamane, 1930. ^ Harrison, 1907 b, 1910. 



^ Harrison, 1907 A. * Ingvar, 1920. 



^ Coghill, 1929. 



